Referees frequently use their hands and body to signal calls like free kicks, offsides, and other decisions, whereas assistant referees communicates with the head referee and the players using their flags.
These signs must be understood not only by the referee, but also by the players. The full list of refereeing signals for the head referee and their deputy referees can be found here.
Here is the complete list of the signals by the head referee according to the Laws of the Game.
Hold a hand or both of your hands up (like in above images) so everyone can see you allowing advantage.
Hold a hand up and point to defending goal.
Raise your hand up to indicate an indirect free kick.
Point to the corner arc using your hand.
Point your hand to the goal area of defending team.
Point your hand or your finger to the penalty mark.
Show the yellow or red card facing the player who is the card for.
Put finger(s) to ear, while extending the other arm/hand.
Draw a square box (TV) in the air using your hands/fingers.
The complete list of the signals by the assistant referee using the flag according to the Laws of the Game book.
Hold the flag over your head with both of your hands.
Direct / Indirect free kick.
1) Raise the flag, give it a slight wave. 2) Indicate the direction of the free kick.
Point the flag to the direction of the throw-in.
Point the flag to the corner arc.
Point the flag to the goal area.
Hold the flag high up to indicate offside on the far side of the field.
Hold the flag in the middle to indicate offside in the middle of the field.
Hold the flag high up to indicate offside on the far side of the field.
Point at the center circle of the field, unless the ball has very clearly passed the goal line.