prop direction: in/out? left/right? CW/CCW?

A quadcopter has 4 propellers. The all look very similar, but every quadcopter has two different type of propellers: two propellers are designed to spin clockwise, and the other two spin counter clock wise.
If all 4 props would be the same, the copter would have no yaw authority and would spin like a frisbee.

So two propellers need to spin CW, and the two others need to spin CCW. But it doesnt stop there. You cannot have two neighbouring props spin the same direction. If you e.g. have a CW prop on the front/left, that means the other CW prop needs to be on the diagonal opposite side, and that determines the position for all 4 props.

When you think that through, installing any of the 4 props determines all other prop position.
There are two possible ways to install the front/left prop, so that leaves two possibilities to install all 4 props "correctly", so that flight can be achieved.

The flight controller NEEDS to be configured to match the prop direction. In betaflight, you can reverse the expected direction with "set yaw_motor_direction = -1".
The default direction for Betaflight and all other MultiWii inspired firmware is: CW on the front/left.

If you have a RTF quadcopter where you cannot access the firmware, you need to know which direction you have to place the props. For all Blade FCs, this is CCW on the front/left.
For all other quads, its best to look at pictures or the manual (yeah, good luck). The easiest way is to take a picture yourself before you crash it ;)

I hope this tip helps you, since you are already here to learn about prop direction, I suggest you also checkout this page about prop installation!