Prop and Opp are the two sides in parliamentary-style debate formats. Prop stands for Proposition and Opp stands for Opposition. The Proposition supports the motion being debated, and the Opposition argues against it. In CNDF, British Parliamentary, and World Schools, every round has a Prop side and an Opp side.
Other Names for the Same Roles
Different formats use slightly different names for the same two sides. CNDF calls them Government and Opposition. British Parliamentary uses Government and Opposition too, but splits each side into opening and closing teams. American formats use Affirmative and Negative. Functionally, the 'supporting' side and the 'opposing' side exist in every format.
Which Side Speaks First
In every parliamentary format, the Proposition speaks first. The Prime Minister (or First Proposition speaker) opens the round by defining the motion and introducing the case. This gives the Prop the first word, but also the burden of setting up the entire debate.