Semaphore is a system of signalling using flags. It can be used to signal between ships and also across open land. 

(table from Scouting for Boys)

It's best to make a good set of flags, with sticks at least 60cm (two feet) long, and lightweight so that your arms don't get too tired holding the flags up.

See this page about the Semaphore Flag Signalling System for more information.