Perhaps you have already received a message telling you that a crate is going to be dropped. Small explanations on this subject.
As already said, you will receive a message announcing the imminent drop. You will see this icon appear on your map:
Rush to the location marked if you don't want a horde of other players to steal the crate from you. You will find red smoke on the spot indicating the location of the drop of the crate.
You will not be the only one to want to recover the contents of the crate. There will certainly be other players, but also trafficking NPCs, who understand well not to have their crate stolen by the first comer, and who will let you know with automatic weapons.
So be sure to get rid of it, then you will have a free hand to grab the coveted content.
Inside you will find weapons and ammunition, as well as snacks. You will also get some RP.
Some crates are advertised as "special", containing more RP, as well as overpowered weapons, such as the rocket launcher, or the minigun.
Small clarification: if you have not yet reached the required rank to be able to buy the weapons you collect, you will only keep them for the duration of the game session. Once disconnected, they will disappear. Whereas if you have reached this rank, you will be able to keep them for free.
A big thank you to Sadicologue for providing me with the screenshots (I apologize for your brutality and for having destroyed my car yesterday: p).