What is a freeroll tournament?

A. Freeroll (sometimes it is spelled free roll) is a tournament where the buy-in and tournament fee is $0. That means you do not need to pay to enter.

The prize pool of the freeroll is guaranteed by the poker room. It cannot be bigger or smaller because add-ons or rebuys are not available.

In most cases poker rooms allow only real money players participation in freerolls. Players must make a deposit first to become a real money player. Sometimes rooms require to play some raked hands to be eligible to participate. Sometimes only some games' winners are allowed. For example Titan Poker and Noble Poker have Winners Freerolls where only TOP 3 finishers in some other tournaments are allowed.

As you see there are lots of ways to enter the freeroll tournaments and all the rooms set some kind of conditions.