On the list of brake parts on Courtesy (rotors, pads, etc.), I saw a caliper seal kit? What exactly is that?

Also, I've had my front rotors resurfaced once before. Can they be resurfaced again or do they need to be replaced?

 

A caliper seal kit is for rebuilding the caliper. You don't need to rebuild the caliper unless it is sticking or leaking. Generally, you can get a rebuilt caliper at a very reasonable price and sometimes the rebuild process can be a PITA.

The only way to tell if you can turn the rotors again is to measure them. These days, the parts store measure them for you and will refuse to turn them if they are below minimum.

 

Rotors which are cut too thin are more susceptible to warping and brake fade. If you are concerned about your rotors get a micrometer and measure the thickness. See Chilton page 9-11 or Haynes page 9-10.

Front rotor, minimum thickness 20mm.
Rear rotor, minimum thickness 8mm.

Front rotor, maximum thickness variation 0.01mm.
Rear rotor, maximum thickness variation 0.02mm.