Givenness is like playing a game of "I Spy" with words. When we use words, sometimes we don’t have to say everything about what we’re talking about because the listener or reader already knows some information about it. That information that is already known is called “givenness.”
For example, if I told you “I am going to the store,” you might wonder which store I’m talking about. But if I said, “I am going to the store on the corner,” the word “corner” gives you more information and makes it easier to understand where I’m going. The idea that you already knew which store I was talking about is the “givenness” in this example.
Sometimes words don't even need to be spoken for givenness to be a factor. In sign language there is an established amount of space between the speaker and their audience, and sometimes signing to indicate something nearby is enough to make the audience aware of what that thing is by using givenness.
Givenness helps us use language more efficiently and with less confusion, because we can assume that the other person has some knowledge about what we’re talking about.