Please explain what it means to ground a club and why it's a penalty?

Grounding a club means that the club has touched the surface of the ground to which the ball lies.

Hence, if it is in a bunker, it is considered a hazard, and by grounding the club in a bunker, it may improve the lie of the ball for the golfer.

Therefore, this happens before the shot is swung.

But, if the club is grounded on grass or tee box, the ball is in play.

Here is the official ruling from USGA site.

13-4. Ball in Hazard; Prohibited Actions

Except as provided in the Rules, before making a stroke at a ball that is in a hazard (whether a bunker or a water hazard) or that, having been lifted from a hazard, may be dropped or placed in the hazard, the player must not:

a. Test the condition of the hazard or any similar hazard;

b. Touch the ground in the hazard or water in the water hazard with his hand or a club; or

c. Touch or move a loose impediment lying in or touching the hazard.

Exceptions:

1. Provided nothing is done that constitutes testing the condition of the hazard or improves the lie of the ball, there is no penalty if the player (a) touches the ground or loose impediments in any hazard or water in a water hazard as a result of or to prevent falling, in removing an obstruction, in measuring or in marking the position of, retrieving, lifting, placing or replacing a ball under any Rule or

(b) places his clubs in a hazard.

2. After making the stroke, if the ball is still in the hazard or has been lifted from thehazard and may be dropped or placed in the hazard, the player may smooth sand or soil in the hazard, provided nothing is done to breach Rule 13-2 with respect to his next stroke. If the ball is outside the hazard after the stroke, the player may smooth sand or soil in the hazard without restriction.

3. If the player makes a stroke from a hazard and the ball comes to rest in another hazard, Rule 13-4a does not apply to any subsequent actions taken in the hazard from which the stroke was made.

Note: At any time, including at address or in the backward movement for the stroke, the player may touch, with a club or otherwise, any obstruction, any construction declared by the Committee to be an integral part of the course or any grass, bush, tree or other growing thing.

Penalty For Breach of Rule:

Match play - Loss of hole; Stroke play - Two strokes.

It's always been my experience that it's much easier and understand the basic concepts of any game by knowing the basic rules of the game you're trying to understand.

Try this > http://www.usga.org/Rules.aspx?id=7788#show=ae24f9... > and then try again....