There are a bunch of different versions of the Gralab 300, with very slightly different labelling for the switches, so at best I'll just run through how I use mine.
There are two power outlets on the timer. One is labelled "Enlarger" and the other is labelled "Safelight".
The idea is that when the enlarger is on, the safelight will be off. When the enlarger is off, the safelight will be on.
The idea is that when the switch is to the right, the timing function will work. When the switch is to the left, the focus function will work - i.e. the enlarger will stay on until you switch it off.
You start and stop the timer using the switch at the top left. You have to set the hands first, of course, to the time you want.
I use my timer for process control only, so the focus function doesn't get used much.