They swore allegiance to you under the tree

I hope to get from you the interpretation of this verse: In The Name of Allah The Merciful The Most Merciful: “Allah was pleased with the believers when they swore allegiance to you under the tree, for He knew well what was in their hearts, and sent down tranquility on them, and rewarded them with an expeditious victory “ (The Holy Qur’an Surah Al-Fath: 18) and who are the believers this verse is mentioning?