what tuning a sub and box is?

Does your sub's manual give you instructions on how big to build the box?  Or instructions for building a vented box with box size and port size listed?  If so, those are tuning instructions.  Measure the available box space in your car to start, then find the recommended box size that will fit into that available space.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

I will dig up the manual and have a look. And what about when people tune it to a frequency? for example  25Hz ??

"Tuning" means designing the enclosure system to optimize a particular frequency or band.  Generally when you see people talking about "tuning" their enclosure to 25Hz, they are talking about a bass reflex (ported) enclosure and the "25Hz" represents the port tuning frequency (Fb.)  For acoustic suspension (sealed) systems, the "tuning" generally refers to either the F3 (-3db) point or to Fc (system resonance) point. Support the12volt.com