If your output is either sourcing current or sinking it, it means the device is actively trying to drive the voltage on that output to one of the supply rails; the positive supply when sourcing, the ground/return when sinking. I.e., that the output is at a low impedance relative to one of the supply lines.
A floating line is one that has a high impedance to the supply/ground system. Floating inputs can behave a bit like little antennas, and pick up random noise from your circuit. This is why unused inputs should either be pulled to +V or ground. Most inputs are high impedance anyway.
If you're connecting standard CMOS outputs to the next devices inputs, there isn't too much to worry about, as the CMOS output stage will have the next device's input driven hard to one or the other logic levels. The output stage has two transistors, one that can drive the output to the +V rail, another that can pull it to ground.
A problem you might encounter, though, is when you have an 'open collector' (OC) or 'open drain' (OD) output stage. These devices basically only have the capability to pull the output to ground. When the output is at logic low, zero volts, the next device's input will be held at ground as the output sinks current. But when the output needs to be a logic '1', the output transistor shuts off, leaving you with .. a floating input. So with this kind of connection, you usually see a pull-up resistor to ensure that the voltage on the input doesn't wag around in response to whatever EMI is at hand. The resistor value is usually toward the smaller end of what you can get away with to not overwhelm the current sink capability of the OC/OD output.
The other common situation is 'tri-state' outputs. These are devices that have two transistor output stages, so they can drive '0' or '1' logic levels without aid of a pull-up resistor, but internally to the device there are controls that can turn off BOTH output transistors, resulting in the 'hi-Z' output condition. If you connect a single tri-stateable output to a single input, and conditions allow the output to go into tri-state mode, you get another case of floating input. You'd probably see a pull-up resistor under these circumstances too, for the same reasons as for the OC device. However, tri-stateable outputs are most often seen in 'bus' situations, where one of several devices asserts the logic level, and all others sit in their hi-Z state. Examine the schematic and there's usually a pull-up resistor on that line somewhere.