The execute command is used to call a command on a specific entity.
From Gamepedia:
Syntax:
execute <entity> <x> <y> <z> <command …>
An example would be if you wanted to strike all zombies with lightning you could use:
execute @e[type=Zombie] ~ ~ ~ summon LightningBolt
So as you can see you can use it to spawn things near entities.
Good luck!
