Drones are combat units that accompany your robot and provide it with different kind of assistance. There's a huge variety of benefits they can offer depending on a type of a drone and its specific customization.
There's even a separate drone hangar deck that can accommodate up to seven units. A drone by itself is not bound to a specific robot, but when deployed in battle, it temporarily connects to a robot you've just selected until the robot is destroyed. In other words, you choose what robot to bind a drone to in battle. You can deploy one drone at a time for your robot but it's not obligatory, and you can deploy a robot with no drone connected. One drone can be used once per battle.
A drone is fully autonomous and is never controlled by a player. It is automatically deactivated once a robot it's connected to is destroyed. It cannot be damaged, targeted, repaired, or be affected by any effects (Lock-Down, Freeze, etc.).
The type of assistance a drone can offer in battle is defined by its ability and microchips installed in it: this is exactly what gives drones unique abilities and effects. Each drone (except for Starter) has a unique role-specific ability that cannot be changed.
Here's an example of a microchip and its function:
Microchip: On Module Use: Damage
After an active module is used, increases robot's output damage for 7 seconds.
This microchip is of damaging type and is tier 1. It can be upgraded to higher tiers, thus granting a greater bonus. There are many other microchips of different types (effects, attack, defence, heal and repair, weapons, shields). The number of microchip slots and a predefined combination of slot types are defined by a drone.
Drones unlock at level 27. You'll get one starter drone for free once you reach this level.
Drones are non-upgradeable;
Drones cannot be used with Titans.