Spring ring clasps have a tiny spring inside, springing them back closed after you open them. Spring rings are popular for bracelets, choker necklaces, and their size works well for jewelry strung with small beads. People normally pair spring rings with either a chain tab, or a medium-large jump ring as the other half of the clasp.
Cheap spring rings have a tendency to be plated shut, and you may resort to using small pliers to open them the first time (breaking through the plating), and then they usually work fine after that. Superior quality spring rings open nicely the first time (and every time after that) and promptly and securely spring shut.