... what does it exactly mean anyway ... in regards to an Xterra or Fronty or other 4WD vehicle?

Solid Axle Swap...Swapping out pretty much the whole front end to make it a solid axle suspension instead of independent. Makes it more...dare I say it..."Jeep-like".

It means to have Solid Axle Swap. So you endup with a straight axle in the front like on a Jeep or 1-ton trucks. The 2 big advantages are extra articulation because the pivot is on the other side of the axle, not at the end of the control arms, second you can lift higher because there are no CV shafts to bind. Now if you want to lift really high, you will either need to drop the diff and/or get front and rear CV driveshafts.