Rigidbody.AddRelativeForce
AddRelativeForce(force: Vector3, mode: ForceMode = ForceMode.Force): void;
Description

Adds a force to the rigidbody relative to its coordinate system.

As a result the rigidbody will start moving.
	// Moves the rigidbody forward along its own z-axis

function FixedUpdate () { rigidbody.AddRelativeForce (Vector3.forward * 10); }
AddRelativeForce(x: float, y: float, z: float, mode: ForceMode = ForceMode.Force): void;
Description

Adds a force to the rigidbody relative to its coordinate system.

As a result the rigidbody will start moving.
	// Moves the rigidbody forward along its own z-axis

function FixedUpdate () { rigidbody.AddRelativeForce (0, 0, 10); }
If you want to apply a force over several frames you should apply it inside FixedUpdate instead of Update.