The handle description says the handle sets are “bolt through fixing”, what does this mean?

Door handles can be subjected to a lot of force which, over time, can loosen and wobble them free.  Bolt thru’ fixing is the best way of fitting lever handles for a long, maintenance-free life span.  Essentially, the bolts holding the handles onto the door pass right through the door AND the lock case or latch case, and fix into the handle rose on the other side.  For this reason, lockcases and latch cases must be “bolt through fixing” too with holes in the right places (most good quality ones are these days, Doorstuff’s ALL are, but if you’re using your existing locks and latches it’s always best to check them).  It is only possible to bolt though lever handles which are on roses, rather than the traditional back plate style, which are always surface-mounted.  Watch this YouTube video about fitting door handles, it shows how the bolts and latch case all fit together to give a strong finished result.