Kekkei genkai are abilities passed down genetically within specific clans.
Kekkei genkai and their related techniques cannot be taught to or copied by others. Although kekkei genkai are usually genetically shared within a specific clan, sometimes it is unique to one person alone, and that even family relatives do not share it.
Generally, all shinobi possess one chakra nature. That's besides the nature they obtain from training. There are different natures, so the more chakra natures shinobi have, the better. That's why most jonin possess multiple chakra natures. However, there are those born with two chakra natures, who are able to fuse the two and create new chakra. They are known as Kekkei Genkai shinobi. For example: Yamato's Wood style is a combination of water and earth. Darui uses a Gale Style, that's a combination of lightning and water chakra.
Above Kekkei Genkai is the Kekkei Touta in which three chakra natures are fused. This is extremely rare and there is only two known shinobi who can use Kekkei Touta. They are Mu the Second Tsuchikage and Onoki the Third Tsuchikage. They use Particle Style that is a combination of earth, wind and fire chakra.