When I look some multimeter manuals it says stuff like "0,2% +5", and in videos I hear this as "0,2% plus 5 digits"...but what does it mean in practice? What is this +5 saying?
The 5 digits is 5 least-significant digits in the display on that specific range. That corresponds to a percentage of full scale, which is the more modern way to indicate accuracy.
The 0.2% should be (but is not always) a percentage of reading.
The total inaccuracy is the sum of the two.
In other words, if the basic DC accuracy of a 4-1/2 digit meter on the 2 volt range is stated as 0.2% + 5 digits, and you are reading a 0.5V signal, the error specification is 0.002 * 500mV + 0.5mV or 1.5mV (+/-0.3% of reading).
A reading of 500.0mV would represent an input voltage somewhere between 498.5mV and 501.5mV.
Edit: Maybe if I show this in a table it will be a bit more clear
Let's assume the range is +/-1.9999 Volts (4.5 digit meter) and the specification is +/-0.2% + 5 digits.
Naturally, you'd normally switch to a lower range (if one is available) once you won't go over the lower range. So if there's a +/-199.99mV range available you'd switch for the 100mV example, but for a 200.1mV input you'd be stuck with +/-0.45% error.