Jabbara,
'Liable to be slippery when wet'
This means that the friction co-efficient of the runway falls below a pre-determined value when measured in the wet, or by a device which artificially wets the runway as it is measuring the friction. A typical friction value (referred to as 'mu') of .7 would be very good, but less than .55 would require the warning as above.
At the moment, there is no official direct correlation between runway friction and braking action. Lots of work is being done at present to try and rectify this situation, including high-speed trials with a variety of aircrft types and different measuring devices. However, it is still felt that in general, a poor 'mu' will result in poor braking action and therefore a longer than expected stopping distance, both when landing and rejecting a takeoff.
For many years, especially in adverse weather such as ice, snow and slush, reduced friction values have been passed to crews who have then adjusted their performance figures to suit. The UK CAA have intimated that until the unified friction value/braking action thing is sorted out, we shouldn't be passing the results of our friction runs to aircrew.
Many runways that have transverse grooves for drainage suffer from rubber buildup and it is this that causes the reduced wet friction. I don't know if this is the case at EDDM but it takes a lot of hard work and considerable expense to clean the rubber out of the grooves.
Cheers,
The Odd One