Lichens are comprised of a symbiotic relationship between an algae and a fungus. The green or blue-green algae produces food via photosynthesis that the fungus can eat. The fungus provides an anchor, moisture, and nutrient source for the algae. Together, they look like a flat greenish coating on rocks.
There are certain insects and such that feed on lichen, but that isn't their true value to an ecosystem. Generally, in the ongoing process of bare-rock succession, the lichen is the beginning of the chain of events that can in some instances lead to a forest. As they live and grow on rocks, lichens produce a weak carbonic acid that is secreted onto the surface of the rock.
This weak acid over a long period of time erodes the surface of the rock into fine dust or dirt. Eventually, after many lichen life cycles, enough soil is produced to allow the growth of moss. Moss continues the process until the soil build up is sufficient to allow grass to grow. The grass leads to small shrubs and bushes, which lead to enough soil to establish tree growth.
Of course this progression isn't even. In many areas you will see all the stages going at one time. There will be a bare rock with lichen growing right next to a mossy patch. A small depression will have grasses growing in it. Nearby, you will note some smaller woody growth. All of this is occurring in the shade of a small stand of trees or at the edge of a forest.