As above, its the width. 2.35 is pretty big so your frame obviously has pretty good tyre clearance.
The thing to remember is this measurement is a very rough guide. Tyres from different manufacturers that are meant to be the same width can vary alot, and also the width of the rim will also affect the shape and width of the tyre.
In mud you want a narrower tyre so your frame doesn’t clog, narrower tyres also generally need more pressure to prevent pinch flats. Silly narrow roadie tyres are 100PSI plus.
Definately watch the weight when looking for new ones…the kevlar beaded versions (folding tyres) are lighter than the cheaper steel bead versions.