The seismic magnitude describes the overall strength of an earthquake. It is to be distinguished from seismic intensity (measurable on scales like the modified Mercalli intensity scale) which more describes the impact an earthquake has on the ground. The current magnitude scale generally used is called the “moment magnitude scale”. Laypeople (including much of the media) may describe this as the “Richter scale” but it is actually a later refinement that is better able to describe large (especially M > 8.0) quakes.
The moment magnitude scale, like other magnitude scales, is logarithmic. That is, a magnitude 5.0 quake is ten times stronger than a magnitude 4.0 one, and a magnitude 6.0 quake is ten times stronger than that (and 100x stronger than the magnitude 4.0 one), and so forth.