It means when you are sitting idling with foot on clutch and try and engage gear the clutch centre plate is spinning meaning the first motion shaft on your g/box is also spinning,hence crunching gears/difficulty engaging gears,problems can be various things,hydraulic system/air in system,faulty clutch cover,faulty centre plate/sticking on splines.Without having to remove g/box,firstly I would make sure your hydraulic system has no leaks and is properly bled,also check for stress fractures at your master cylinder mounting,good luck
cdc
November 2012