The meaning is literally 'spit':
> Spitting for baseball players is like blinking; it's going to be a tough habit to break. It's not even just spitting on the ground. You know, a lot of us have routines and habits — you know, you spit a little bit in your glove before a pitch. It's got to be a conscious effort from everyone. And, you know, we're going to have to do our best. The Current
> Saliva is also used as a lubricant to break in a stiff new baseball glove. Though studies prove that the practice is bad for the glove, both pros and amateurs continue to spit into their glove to soften the leather. The Indian Express