You can safely ignore the ”c” and buy the wheel you were considering.
559 is the Bead Seat Diameter - where rim and tire overlap - in millimeters.
The 21 - and the 19 - is the rim width in millimeters.
While most prefer to use matched sets, a 2mm difference in width isn’t going to hurt anything for casual riding.
If mismatched, I’ve heard people prefer the widest to go at either end.
Some prefer rear, to get a bigger air chamber, a bit more pneumatic suspension out of the rear tire.
Some prefer the front, as keeping the front wheel gripping is more important than the rear.
As for tires 2.1” is a very common width. If you ride mostly easy, smooth trails, feel free to go a bit narrower. If you like to ride bumpy, lumpy or soft surfaces, make a guesstimate to how much wider you can go.
I like the Conti Race Kings, but that might strike you as excessive for the planned use.
Schwalbe Smart Sam is good value for money.
Be wary of 26”. There is something like 5 different and incompatible wheel sizes using 26” as part of their name. The millimeter size is easier to get right.