Suppose you grab a typical \$9\:\text{V}\$ battery and attach a \$1\:\text{k}\Omega\$ resistor across its terminals. A schematic of this arrangement might be drawn to look like this:
simulate this circuit – Schematic created using CircuitLab
To achieve the circuit, two wires were used. (In electronics, these are often called nodes rather than wires. There's a reason for that but it takes a little time to fully apprehend the whys.)
What's the voltage on these wires? Ask yourself that question and think closely about it. Is \$\text{Wire}_1\$ at \$0\:\text{V}\$? Or is it at \$-9\:\text{V}\$? What about \$991\:\text{V}\$?
There's no way to make a good argument here. No matter what value you pick, someone could argue a different value.
So far as the circuit cares, though, all it needs to know is that \$\text{Wire}_2\$ is \$9\:\text{V}\$ more positive than \$\text{Wire}_1\$. That's all that's needed in order to figure out what the circuit does.
(I'm going to shift over to calling things nodes, not wires, now.)
Part of the point here is that voltages are always measured between two nodes. There is no meaning to a number applied to a node (in terms of applying the idea or units of a voltage to it) if it is not applied with respect to some other node.
There's another point I can now make. I can choose any one of the nodes and consider it to be the implied reference for all numbers I write elsewhere on the schematic. If I choose to assign such a "special wire/node" for this purpose, then it is called the "ground" reference. Then, when I write a number somewhere on the schematic or talk about specific values, others know that these are in reference (with respect to...) this assigned "ground" reference.
I can choose any one wire or node for this purpose. But only one, of course.
(This is not to be confused with an Earth ground, which is that metal stake in the ground your picture shows. That's a different meaning, entirely. Though it definitely has implications for the choice a designer would make when selecting a wire or node for a reference point in the schematic!)
Now, we could put a giant metal stake in the ground and do this:
simulate this circuit
If you were standing bare-foot on the dirt, your body would be likely making a galvanic connection to the dirt. And if you reached up to touch \$\text{Wire}_1\$ with a hand, you'd know it quick because your body would suddenly create a galvanic path from \$\text{Wire}_1\$ to the dirt and you'd experience a shock as a result. How much of one would depend on your body's resistance, the contact resistance of your feet to the dirt, and the resistance going from that point in the dirt over to where the stake was placed. But very likely you'd feel it.
So in this case, you would want someone to label \$\text{Wire}_1=1000\:\text{V}\$ for you and not choose to call it \$0\:\text{V}\$!
It turns out that transformers, can develop a voltage between two wires just like the first battery circuit I mentioned, except that it will be AC instead of DC. But the transformer wires don't have to be galvanically connected to anything. They can float.
Suppose a transformer pole is located at the bottom of a hill and your house is at the top of that hill. The transformer on the pole supplies "mains voltage AC" to the home without touching any of the dirt in between the two locations.
Because of the fact that different parts of the Earth (and air in the atmosphere, too) can, for example because of thunderstorms, be at very different potentials with respect to each other (the bottom of the hill with respect to the top of the hill), it's possible that touching either side of a plug or the metal chassis of a toaster oven when standing on some dirt on top of the hill might create a "galvanic circuit":
simulate this circuit
This would NOT be a good situation to be in.
A solution is to do this:
simulate this circuit
That helps because now, if the hill top "tries" to acquire much of a voltage difference versus the hill bottom where the pole is at, it "shorts out" through the two grounding rods stuck into the dirt at both ends. This helps to make it safer (not perfection, but safer) by keeping the dirt at the hill top near the home closer to the same voltage as the dirt at the bottom of the hill near where the power line transformer sits.
This is also called a ground. But this is an Earth ground. And it serves an entirely different purpose and therefore a different meaning.
Now, it turns out that electronics designers are aware of this safety issue and they use this knowledge when designing devices you might purchase. And it helps other designers, when reading these schematics, to make judicious and careful choices of what wires or nodes within a schematic are labeled "ground."
So there is a reason why these terms may become "conflated" a bit. But they really need to be kept separated in your mind, despite that English users may not be as clear as you'd want when they say "ground."
