A pearl necklace is the result of a man ejaculating onto the upper body of his partner, especially the neck. The result - if he has enough cum - is supposedly similar in appearance to a necklace of pearls as it forms into sticky white globs. Pearl necklace is also sometimes used to refer to the act of a man cumming on his partner's neck.
The way that the man was made to cum is irrelevant, but a tit-fuck will often cause a pearl necklace with little further effort. However, it could be the result of vaginal or anal sexual intercourse (if the man is quick enough on the move), masturbation by himself or the partner or oral sex.
For a man (especially but not exclusively a dominant man), it can be highly psychologically satisfying to come on a girl's face (or some other part of her body); for her to have that as a potentially visible sign, even if it is never actually seen in public, can make him feel virile and desired. Many believe that it is an expression of great love and affection bestowable upon a woman by a man.
Some girls wear a man's cum on them proudly, either because they consider it a show of their own desirability or because they feel that a pearl necklace has the look of beautiful jewellery.
In counterpoint to ejaculating on her neck or breasts - creating a pearl necklace, cumming on a woman's face is commonly known as giving a facial. (If many men do this simultaneously or nearly so, it is called bukkake.)