A DKIM selector is a symbolic name that can be given to a DKIM record. This can be used to differentiate between several DKIM records. It is the name that appears in front of the ._domainkey part, for instance key._domainkey. In your DNS records this will look as follows:
The value of the DKIM record contains the contents of the public SSH key after p=. SSL keys do not expire (unlike SSL certificates), but for security reasons it is advisable to replace your public and private key from time to time (for example every year). We show a method of how to do this without downtime in our Exim DKIM manual.