If something is off topic it is not related to the subject or topic that it is supposed to be discussing.
There are a number of different situations where you might encounter this phrase, but it basically always means the same thing. For example, if I assign a student to write an essay on freedom of speech and he starts to write about how the US government is spending too much money, I will tell him this is off topic. He was supposed to write about freedom of speech, not about how much the government is spending. Someone can also introduce a subject that is off topic in an oral discussion.
yea, Mr.pohnpei397 said it with detailed..i am also agree withi his opinions...off topic means it is completely out of desired topic. If yoiu are assigned to write about a global warming and you are started to write about faorite musical shows, it is said to be off topic...it is completely another track...so i hope you understood what i meant...