This entity is used to provide players all sort of weapons, ammo, health or other items; in short, this is the entity you should use whenever you want to give something to a player.
You have 2 ways of using this entity: with a spawn location entity to let a player spawn in a game with a set of items (ie, the enviro item, aka Battle Suit) or with a trigger_multiple entity to distribute items during the game. In any case, the same technique applies: link the trigger (or spawn item) to the target_give and then link the target_give to any items you want to provide.
The goal is to have a set of items provided upon (re)spawning.
The technique is fairly simple: in a 2D view of your map, right-click near
the spawn entity and choose the
target -> target_give entry.
Deselect everything and then select the spawn location item (shift-clic in the 3D view)
then the target_give entity and press Ctrl+k.
Once this is done, add any weapon or item you would like to give at respawn near the target_give entity, deselect everything, select the target_give then select the item and link them using Ctrl+k. If you want to provide more than 1 item, you will have to repeat this for each item.
The goal is to have a set of items provided during the game.
This is basically the same operation as with the spawn location; create a trigger_multiple brush somewhere in your map and add (right-click) near the trigger a target_give entity. Link them (select first the trigger then the target_give), then add any items you want to give to players and link each one of them with the target_give entity (first the target_give then the item).
You can always check that you have linked everything in the right order by looking at the colors of the links beetween a target_give and an item.
Note also that you can always find this info in Radiant by looking at the properties window of a target_give entity (select it then press n to display the properties window).
You may also check the example map with some target_give entities.