Command signs are blue discs with white symbols. There are two kinds: signs that specify an action all drivers must take, and signs that specify which class of road user must use a particular part of the roadway that others may not.
Action commands include switch headlamps on (dipped beam), pay toll (the T: pay or take an alternative route), minimum speed (a number on blue means you may not drive slower than that; a number on a red-bordered white disc is a maximum), keep left / keep right of an obstruction, proceed straight / left / right only (straight arrows apply at the sign itself) and turn left / turn right at the next junction (bent arrows refer to the next junction). The roundabout command means travel clockwise and yield to traffic already in the circle.
Class commands show a vehicle or person on blue: taxis only, buses and minibuses only, goods vehicles only, pedestrians only, pedal cyclists only, cyclists and pedestrians only, and mass-based commands such as vehicles over 10 t must use this road. These reserve that part of the road for the class shown.
A red cross over a command sign is a de-restriction sign: the command no longer applies (end of headlamps-on, end of toll road). Temporary command signs at roadworks have a yellow background with black symbols and carry exactly the same force as permanent ones.
The common trap in the test is confusing a blue disc (you must) with a red-bordered disc (you may not) showing the same symbol, and confusing a straight arrow (here) with a bent arrow (next junction).