Semantic Versioning, or SemVer, is a relatively new specification for how to number software releases, which uses three numbers (e.g. 1.2.6) instead of two (e.g. 1.2). It is being used by more and more open source projects, including Drupal, as of Drupal 8. This presentation walks you through the numbering system so you can understand how to read and apply the version numbers correctly, along with understanding why more projects are adopting it.
Note: There is a typo in the slide that shows pre-release versions. The slide shows a period as the separator, when it should be a hyphen. I.e. 1.0.0.alpha1 should instead be 1.0.0-alpha1.