Looseness most commonly occurs in older carpeting, especially if it is one large piece of carpet used throughout several rooms. Over time, the material in the carpet loses elasticity and stretches out. The problem becomes worse if the adhesive used to fasten it to the floor loses strength or if the carpet is pulled up from a corner in which it had been tacked down. A loose carpet leads to rolls. Not only are they trip hazards but as they get worse they are also unsightly.
When you re-stretch a loose carpet, you pull the slack out of the carpet so it once again lies flat against the floor. Trim excess material away and tack it back down to keep it in place. Because loose carpet often occurs on very large pieces of carpeting, you might need to rent specialized equipment to not only help you take up the slack but also hold the carpet in place while you trim and refasten it.
You shouldn't need to re-stretch carpet at all. Good quality carpet that is properly installed should stay in place. If you do see rolls developing in carpet and decide to re-stretch it rather than replace it, you should need to do the job only once. The carpet should then lie flat until you decide to replace it. If the carpet continues to develop rolls, either hire a professional to re-stretch it or install new carpeting.
If you decide to re-stretch carpet on your own, the equipment you might need to rent is available at home improvement stores. Using knee pads will make the work more comfortable. If the padding under the carpet is damaged or if the carpet tears while being stretched, it is time to replace both. Careful installation of the new carpet will ensure you don't have to stretch it in the future.