A proposition would be something like, It is raining. Because it is true or false, you can either affirm that it is raining, or deny that it is raining. You usually do this by going to the window and looking outside. If you see rain coming down, you can say that the proposition is true. If, on the other hand, the grass is as dry as the sand dunes in the Sahara desert, the proposition is false.
Propositions are typically declarative sentences.
Babe Ruth played baseball. Stephen cut Angela s hair. Killing is a sin. Jack and Jill went up the hill. America is the richest nation in the world.