There are two types of shields in the game: physical shields and energy shields. They both make your robot more durable but have different characteristics.
A physical shield is a solid barrier that protects your robot from incoming damage. It has its own durability and can be destroyed. It is effective against energy weapons, less effective against kinetic weapons (those deal double damage to physical shields). Splash projectiles and flamethrower weapons bypass the shield.
There are several types of energy shields: regular energy shields, Aegis, Absorber, Reflector.
Regular energy shields can only block kinetic damage; they also can regenerate.
Aegis Special energy shield impenetrable against any weapon type (except for radial explosion damage).
Reflector partially blocks received damage and redirects a fixed percentage of it to the one dealing it. Reflector does not fully protect the robot, however, it cannot be mitigated.
Absorber is an impenetrable shield that can absorb an unlimited amount of damage to power-up built-in weaponry.