Organisms, both animals and plants, compete with individuals of another species or with their own species for resources. These resources may include food, space, etc. A competition for resources is essential, since they are in limited supply and only those organisms who can successfully compete for them, may survive. An example of a plant that competes for space is cactus in desert regions. In those harsh conditions, plants compete with each other for moisture and water they can gain from infrequent rains. Cactus plants space themselves and send out roots in all the directions to cover as much area as possible. The wider the root spread, the higher are the chances of getting moisture (or water).
[Plants] need space in which they can get enough light, and water containing minerals. (Richard Steane, Ph.D., "Competition")
Comparable examples in animal kingdom include mussels and barnacles.
Hope this helps.