An SIP account is used to connect a client's VoIP phone (or software like Axon) to a VoIP service.
SIP itself is a protocol or set of rules used to communicate between voip devices. The account is just a series of login credentials (e.g. username/password) to authorize that communication.
SIP accounts are configured with Axon as 'External Lines' and can be set to ring to a particular extension or group of extensions on incoming calls.
Axon can also be set to use an SIP account / External line to make outbound calls.
In most cases an SIP account is linked to a single phone number.
By having 1 SIP account for each phone number, Axon can be set to handle and route the calls differently for each line.