Show signals on the drive side is a advanced setting, accesible in the Advanced Settings/Construction window.
When enabled, this option makes all signals appear on the same side as is set for vehicles to drive on in Game options window.
Normally, signals on train tracks appear on the opposite side of the track to which vehicles drive on the road, i.e. if you have it set up so that vehicles drive on the left, any signals would appear on the right of the train track.
With this setting enabled, the situation is reversed and signals appear on the "driving side", i.e. if you have it set up so that vehicles drive on the left, any signals will also appear on the left of any tracks you may have. This setting has no effect on how the game works, but it makes it a bit more realistic.
Example: If you use the DB Set or the DB Set XL, you can enable this option because in Germany road vehicles drive on the right side of the road and the train signals are also on the right side of the track. On the contrary in Italy train signals are on the left side and road vehicles drive on the right one, then an italian player may prefer to disable this setting.