"Groundcover" is a loose term applied to a plant used to provide vegetative cover over broad areas. They're useful in situations where lawn might be impractical (e.g. under trees or other shade, steep banks, awkward spaces) or a more aesthetically interesting alternative is desired.
Without regular mowing, and the possibility of using plants that produce flowers, seeds or even fruit, groundcovers may even be considered more environment and wildlife friendly than lawn.
Groundcovers may be considered low-maintenence in the sense that they don't require mowing, but note that they will probably need some maintenance to keep them tidy, lush and healthy.
If the groundcover is insufficiently dense, weeds will have an opportunity to grow, which could be difficult to extricate from the groundcover once established. therefore, selection of a species suited to the conditions plus adequate water and nutrition will be critical to achieving a satisfactory result.
A variety of plant types can be employed as groundcovers, including densely-planted ornamental grasses and dwarf, clipped shrubs. Most commonly, though, plants which are naturally low-growing and with a running or spreading habit are employed as groundcovers. In many cases, this habit means many groundcovers can also be used in hanging baskets or draped down banks or retaining walls (depending on their vigour and sun-tolerance etc), while others may be trained up supports as climbers.
In some plant groups like roses or grevilleas which are usually upright shrubs, certain species or varieties are suited for use as groundcovers (and may be especially bred for this purpose)