Calling supply drops becomes available since you visit the Mother Base for the first time, after completing the second main mission (Diamond Dogs). It's a very useful option thanks to which you won't have to worry that suddenly you will no longer be able to use a weapon with suppressor or that destroying the tank found in enemy base will be impossible. In order to call a supply drop you must activate the iDroid device. Go to the Missions tab and then select the first option from the list - the Supply Drop. The drop that you should use most frequently is Ammo/Fulton. It allows you to replenish the weapon ammunition and the supply of Fultons. The best thing is, it regenerates the suppressors in your silenced weapons (by default, the suppressors are available in your assault rifle and in the tranquilizer gun). Use two other supply drop options only if you have selected the wrong equipment before the mission and because of that now you have trouble with dealing with the enemy. Calling those drops is much more expensive.
After selecting the supply drop type, you must choose a landing zone. It should be close to you and far from the enemy. Now all that is left is to wait until the chest with the equipment reaches the ground (make sure not to position yourself directly underneath it!).