A fixed-term membership is a membership for a set duration of time – i.e. a 12-month fixed-term membership is a membership that starts from the moment of purchase and ends in 12 months' time.
To purchase this membership, you will make a one-off payment at the point of purchase that is the cost of the duration upfront. This often saves you money in the long run and allows for better budgeting as the money has come out upfront.