A named user is a "named" individual accessing the software. Only that person can use Fitrix under that name. If you have 10 different employees that use Fitrix, you need 10 user licenses. If an employee stops using Fitrix you would access your account management screen to retire (or pause) that named user. 

A concurrent user is any user accessing the software. You will need licenses for the maximum Fitrix users logged in at one time (concurrently) . For example, if you have 10 different employees using Fitrix and there are times when all 10 are using Fitrix at once, then you need 10 concurrent user licenses. If you have 25 users of Fitrix but some are light users or occasional users such that there at most 15 users logged into Fitrix at the same time (concurrently), then you need 15 concurrent user licenses.