*sights* Manual, gentlemen, manual... It simply can't be layed simplier than it already is in the manual.
Ready is an action taken as a preparation for the next action - after a ready unit can do one of the following:
- re-roll dices on weapons w/squad trait during the next fire action,
- fire a weapon w/ ready trait
- perform a special move (tunnel, climb, jump etc)
- perform any special action specific to the unit (like uncloaking the Infiltator bug)
- set up heavy weapons (in case of MI microsupport platforms)