A "sniff kiss" is the traditional Thai romantic kiss. You plant your nose on your loved one's cheek (or other part of the body) and gently sniff in.
Thai people consider this very sweet and romantic. It is also used by parents to show affection for their children.
In general, men tend to "sniff kiss" their lovers more than women do.
Lips on lips western style kissing is a relatively recent introduction, but is increasingly popular between lovers.
Nevertheless, if you learn to "sniff kiss", your Thai lover will certainly appreciate it and be very pleased!