There are five special points where a small mass can orbit in a constant pattern with two larger masses (such as a satellite with respect to the Earth and Moon). The Lagrange Points, named in honor of Italian-French mathematician Joseph-Louis Lagrange, are positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. This mathematical problem, known as the “General Three-Body Problem” was considered by Lagrange in his prize winning paper (Essai sur le Problème des Trois Corps, 1772).

The five Sun–Earth Lagrangian points are called SEL1–SEL5, and similarly those of the Earth–Moon system EML1–EML5, etc. Orbits around Lagrangian points offer unique advantages that have made them a good choice for performing certain spacecraft missions.
For example the Sun–Earth L1 point is useful for observations of the Sun, as the Sun is always visible without obstructions by the Earth or the Moon. SOHO, the ESA/NASA solar spacecraft is positioned there.

read descriptions about invidual L-points here