"I guess..." can be used to announce a decision that you're not sure about, or not enthusiastic about. In the example above, you're not very excited about cooking the frozen steak, but you don't have anything better to cook so you say "I guess..."

As another example, imagine that you're having dinner with your extended family. People are working on cooking, and you don't have anything to do so you say:

I guess I'll set the table.

"Setting the table" means that you put all the dishes, glasses, napkins and silverware out on the table.

Another similar use of "I guess" is to agree with someone while showing that you don't completely want to agree. For example:

A: Are you OK with Chinese food tonight?

B: I guess so.

And "I guess" is also useful for showing someone that you've changed your mind about something:

I guess I might as well stick around and have one more drink.