Near the coast you see a cannon cursor. This allows the fleet to establish a site for a later landing of army forces. When you assign fleets to this mission, you are accepting the potential of automatic interceptions by enemies. Any enemy fleet that enters the sea zone or patrols in the sea zone can find and intercept your forces. You must be prepared to defeat these attacks. 

You cannot move land forces on the turn the landing site is established. Land forces can be moved to the selected enemy province on any subsequent turns as long as you maintain the landing site by keeping ships in that sea zone.