Something linear is like a line. The adjective usually refers to something that follows an expected order or sequence — like railroad tracks or even the progression of a disease.
Drawn from the word line, something linear follows that concept, coming from the Latin līneāris, "belonging to a line." You might come across the word in mathematics, as in a "linear equation," or in electronics, with a "linear circuit." It also can be used to refer to a written work that follows a "linear narrative." If a movie has lots of flashbacks and happens out of order, it does not have a linear narrative. You might find such a movie puzzling but enjoyable, or you might prefer something more chronological and linear.