"To kiss someone gently".
It is more often used to describe a kiss that is done quickly and abruptly, possibly in a
cute way. It can indicate both romantic, parental and, possibly, friendly
love.
It comes from the
Russian word "чмок", meaning the same thing. In English, it can be both a noun and a verb (chmoking, chmoked).
1. "He chmoked her gently on her
bellybutton"
2. "The mother chmoked her
daughter on her head when she was heading off to school"