"Having the floor" indicates that someone has been allowed to make a speech before an assembly. As the definition says, this this is governed by the Rules of Order (or various other names for the same thing) which may limit the length of the speech.
Ben Jackson's comment is right on target. Only one person can have the "floor" at a time. In order for someone else to speak, the person who has it must yield the floor to someone else, either voluntarily or when ordered to do so by the leader of the assembly.
Again, the rules from this will vary between different assemblies: how much time someone may take on the floor, who the floor can be yielded to, whether the other members should stay silent when someone has the floor, who can order someone to yield the floor, and so on.
For example, in the US Senate, a senator may have the floor for an unlimited amount of time. This allows for a legislative tactic called the filibuster where one senator can block legislation by continually talking. Rather than waste time on useless dialogue, the Senate leader may choose to instead deal with less controversial legislation, or kill the legislation entirely.
A filibuster can be stopped by something called cloture, which (more or less) requires a two-thirds majority to pass. While on a filibuster, a senator must remain standing and must talk more or less continually, although they may temporarily yield the floor for "questions" (which might go on for some time, to allow the filibustering senator some rest). A senator also may not leave the Senate itself, which historically has led to some creative solutions with regard to things like bathroom breaks.