what is an actuator in robotics?

The key decision is stiffness versus softness. A rigid geared actuator is precise and strong, but hits hard if it finds you in its path: it is the muscle of the caged factory. To live among humans, designers trade in elasticity: the Agility Digit inherits from locomotion labs springs that absorb and return energy like tendons, and the 1X Neo takes the concept to its domestic extreme with tendon-driven actuators that yield on contact, which lets it weigh 30 kilos and walk past a child.

When a humanoid brags about lifting weight, the engineering question is at what price: powerful actuators are heavy, drain batteries and run hot. Every degree of freedom adds one more muscle to feed, which is why serious robots are designed from the task, not from the record.