It's a way of expressing that you're not sure about something, but you're leaning towards the negative answer. It conveys a sense of uncertainty or resignation.
Here are some examples of how it can be used:
* "¿Vas a la fiesta?" "Supongo que no." ("Are you going to the party?" "I guess not.")
* "¿Te gusta la comida?" "Supongo que no." ("Do you like the food?" "I suppose not.")
* "¿Crees que va a llover?" "Supongo que no." ("Do you think it's going to rain?" "I guess not.")