The term "free speech" describes a legal system in which a person cannot be stopped from saying anything he or she wants, as long as the speech doesn't cause immediate harm to those around him or her - even in the freest society, it is still illegal to shout "Fire!" in a crowded movie theatre.
The benefits of free speech are many. The citizens may debate their government's policies and actions fully, without fear of punishment. A free press allows the citizens great access to news and information, allowing them to more accurately judge the events taking place in the world around them. Tyrants and despots rightly fear free speech more than almost anything else. On the other hand, free speech may be hate speech; it may be lies. Such speech must be allowed: once a government is allowed to determine which speech is to be allowed and which is to be suppressed, the temptation to abuse that power is overwhelming, and speech isn't free any more.