This is open to interpretation, but there's a couple of more obvious reasons for this behaviour.
In part, particularly for younger horses, this is a modified grooming action. My young horses do this and even nibble on my hair when I'm scratching their necks or backs, as if you watch horses mutually grooming, that is what they are attempting to do to me. With the one exception, they have been taught not to use their teeth on people. My rising 6yo mare does this with me.
They also sniff as a way to gain information. Where you have been, who and what you have been mingling with. Such busybodies! Horses will greet each other frequently and sniff as a way of checking each other out. Particularly given we humans put so many different scents on us throughout the day with a showering and deodorant routine. I avoid wearing perfume of body sprays around my horses and try and use a non-invasive smelling deodorant.