Suppose can be used primarily in two different ways. One is similar to I guess, or affirming something. For example "Did you like the food?" "I suppose." The other way is to introduce a hypothetical situation, almost similar to the phrase "think if..." So, for example, if you are introducing a hypothetical (hypothetical, if you aren't farmiliar, is an adjective used to describe something made up but possible) situation, you would say "Suppose this were to happen". I hope that explanation made sense :)