Answer:
The Filibuster basically says that as long as a member of congress wishes to continue to debate, they can. This means that other members of congress have to listen to them for however long they are talking, and they can talk about anything they want.
This was not as much of a problem when our country was first created because they didn't have more than 30 or 40 (I'm not sure on the specifics) members of congress, so debates eventually ended at a reasonable time.
Now that we have 435 representatives and 100 senators, debates last longer and there is no way to stop the debates or move them along in an orderly fashion.
When a senator or representative continues speaking for an excessive amount of time, this is what is called filibustering.
I'm not a legal expert or anything like that, that's just my understanding. I hope this answers your question.