In Movie Credits what does Key grip mean?!

In Movie Credits what does Key grip mean?

Also what does grip and second grip do?I allways seen this in credits and never knew what it ment.

Scouts the location if any rigging or special problems are anticipated. On the prelite (prior to the shoot) the Key Grip loads in the equipment and sets up the stands to support lights or any other hanging elements, sets up dollies and tracks, cranes, sets, etc. On the shoot, the Key Grip operates the dolly and/or crane, and moves any heavy equipment and loads in and out. Other grips may assist the Key Grip in these duties. Such individuals are in Category Three.
www.mca-i.org/articles/irs/glo...
AKA: Key-Grip The chief of a group of grips, often doubling for a construction co-ordinator and a backup for the camera crew. Key grips work closely with the gaffer.
www.ldsfilm.com/ar/glossary.ht...
In American film-making, the key grip is the chief grip on the set. Like a foreman, the key grip directs a crew of grips, some with specialized skills such as dolly grips, crane operators, camera car operators, etc. Grips' responsibilities bridge several departments on a film set including camera, lighting and art departments through the use of rigging, as in unusual camera positions, overhead lighting positions and overhead set dressing.
en.wikipedia.org/wiki/Key_grip