This article describes the simplest way to enable MACSec using preconfigured static key-string. The example was tried on Catalyst 3850 and should work on other switches too.
There is another article that I wrote years ago which describes a more complex implementation with dot1x etc.
Media Access Control Security is the way to secure point-to-point Ethernet links by implementing data integrity check and encryption of Ethernet frame.
When you configure MACsec on a switch interface (and of course, on the other switch connected to that interface), all traffic going through the link is secured using data integrity checks and encryption.
Data integrity is done by appending 8-byte header and a 16-byte trailer to the Ethernet packet which is generated before a data is sent and checked upon receiving on the other switch to prove that the data inside the frame was not modified on the way. If the check fails, the packet gets dropped.