Grounding is a meditation technique used by those who practice shamanism, paganism, and various other forms of magick. Depending on who you talk to there are various opinions about what grounding is, but there are several universal points:
Grounding involves centering yourself and focusing on one idea, thing, person, etc.
Grounding disperses negative (usually psychic) energy and draws in positive energy from another source.
Most ritual magick begins with grounding. Solitary workers find it a useful tool for improving their ability to concentrate on their objective, while groups or covens use grounding to make sure that everyone has the same goal and is starting the ritual from the same page.
There are multiple grounding methods. Some people imagine themselves as trees, with their roots deepening into the earth to find security and nourishment. Others prefer to see themselves in a white, blank space and to hold themselves there for as long as possible. As long as a clearing and focusing of the mind occurs and everyone involved is using the same technique, how you decide to ground is irrelevant.
Please note that there is grounding etiquette. If you decide to draw energy from an object or to cast your negative energy into an object, it is considered spiritually rude to do so without first asking that object's permission, i.e., "Mr. Tree, may I please draw from your life source?" To do otherwise is to risk pissing off whatever object you're trying to work with and having your spiritual energy fucked with.