What do you mean by "top up my expansion tank by about 0.2 Bar"?

"Tony" wrote in message
oups.com...
I have a very small leak in my radiator pipes because every 2 weeks or
so I have to top up my expansion tank by about 0.2 Bar.Is it possible
to put some Leak Fix product into my boiler system bearing in mind that
I have a 6 month old condensing combi boiler and I cant locate the
exact place of the leak?