They are statements to which we add a final "tag" or question. For example:
You speak English, don't you?
The movie wasn't that bad, was it?
He can join us, can't he?

Many learners do not like using tag questions since they can be more confusing than regular questions (which, let's face it, are already damn tricky on their own!).
Even so, it's important to give tag questions a shot. They can really take your English conversational skills to the next level.
Using tag questions can:
show that you are listening and interested in what the other person is saying
confirm or deny information quickly
keep a conversation going
make you sound friendlier and more confident.
If you're confused about how to use tag questions, check out my new YouTube video. You'll learn why we use tag questions, how to properly form them, AND how to respond to them.
FYI*
Check out the meaning of the expressions/words in bold that I used in this post:
Let's face it (expression): to admit that something is true or real
tricky (adj): difficult to do or deal with
give something a shot (expression): to try or attempt to do
* fyi: This is an abbreviation of "for your information" which is another way to say "just so you know."
Happy studying!
Gabbie