It's saliva. Not heat related. It can be a good thing (chewing on the bit, giving in to the bit, using a different flavored/textured bit than normal like a copper bit, having a soft mouth) or a bad thing (a result of fighting the bit, pulling, chomping on the bit, a hard mouth, too much bit)
Clear as mud. Look at how the rest of the horse is acting
Relaxed, or fighting the jock?