Presuppositions - assumptions - underlying beliefs. Any of these terms will do if it describes for you the wealth of decision making that goes on before you become conscious of making a decision. Sometimes we use terms like instinct, experience or knowledge to cover much the same ground. They all amount to saying something like "I didn't have to think about it. I just knew" or "I don't have to think about that because everyone knows that."
It won't take you long to notice that these underlying decisions are based on context: it's remarkably hard to generate a statement that is true across all contexts (even that one can be challenged, for instance by people who will argue that 2 + 2 is always equal to 4). Try it now. Make a completely obvious general statement and then notice the contexts where it does not apply.
When everything is going well, we do not need to think about presuppositions - that's more or less the point. When we hit a snag, it's time to notice that a part of what is applying differently is our presuppositions. Begin by uncovering and thinking about your own. When you can consciously replicate those decisions you made before thinking, then you will be in a much stronger position for working with the presuppositions other people are bringing to the party.