In addition to what Mike said, in your grbl settings $20 enables / disables “soft limits”. So, $20 = 1 will enable soft limits that won’t allow you to jog past the limits of your machine. This is assuming your machine is homed, your max travel settings ($130, $131, $132) and steps/mm ($100, $101, $102) are accurate, and there are no mechanical issues. No harm in enabling the soft limits if the other settings are accurate. The controller will enter a feed hold state and then alarm as soon as it receives a command that would take it beyond the limits.
$21 = 1 will enable hard limits. Basically that will turn your homing switches into limit switches. The problems with hard limits are:
- You lose position, and the machine must be reset and re-homed.
- Physical switches are prone to EMI, especially whn the spindle and dust collection are going. You may receive false detections.