The pressure gauge indicates the pressure in the primary circuit system and gives an idea on whether the system needs pressurising or not.
The pressure gauge which is normally located at the front of your boiler should read around one and a half bar (1.5Bar). If it is below one bar then your boiler pressure is too low and you should top up the pressure if you know how to do this by way of the filling loop. If not or it continues to drop you have a problem and you should call a gas safe engineer.
Essentially if your central heating is losing water pressure that usually means it is losing water. This could be due to a leak somewhere in the system. So have a good look over the entire system including all pipework and radiators. Places where different parts of the system are joined together are most often the likely culprits. Another culprit could be the Pressure Relief Valve.
If you have an issue here its best not to ignore it as it will no doubt cause more problems down the line. Contact a gas safe engineer like ourselves to rectify the problem.