When the synchros in the gearbox are on their way out, it'll be harder and harder to get the car into gear.
Synchronisers are basically used to get one part of the gear up to the speed of the other part of the gear before engaging. In that way, there's no more need to double clutch as one did in history, but you have an effortless gear change.

This is the synchroniser assembly for the Rover PG1 gearbox' 3rd and 4th gear.