Suppositions are ideas that you think are true, even if you don't have all the facts.
Suppositions are just guesses, so they might not be right.
Examples:
Her supposition was that the cake was delicious.
I made a supposition about where the cat was hiding.
His suppositions about the treasure were all wrong.
My supposition about the weather was totally off.
Their supposition about aliens visiting Earth was very silly.
The supposition that dogs can talk is very funny.