To back-burner something is to put it off for now, because other work needs you first, or because it can wait. The picture comes from a cooker: the rings at the back are where you leave a pot to cook slowly while you watch the ones at the front. People at work say it a lot in American English, as a verb (“Let’s back-burner this”) and in the older phrase “put it on the back burner.” It is informal office talk, not a kitchen instruction. You can bring the same thing forward later if it becomes urgent. Though actually, “put it on the back burner” is still the more common way to say it; the short verb feels newer and more American.