After he developed his GR, Einstein attempted to see what his theory predicts for the universe as a whole. In the curved space-time there is a possibility to construct a universe with positive curvature, which will be finite but without an edge. Apparently, Einstein did not like an idea of an infinite space-time.
His model had only one problem: since gravity is an attractive force, such a universe had either to expand or to contract, but it could not stay still. But astronomers of the time knew that the universe (i.e. Milky Way) was not expanding or contracting.
Thus, Einstein decided that his equations were wrong. Later he called this his ``greatest blunder''.
He decided to correct his equations by adding a new force that was not known before. This force should be repulsive, it should compensate the gravitational attraction. He called it the cosmological constant, because the energy that was producing this force was constant everywhere.
Thus, he was able to construct a stationary model of the universe. This model had only one problem: it was unstable.