A person who identifies as transmasculine considers themselves masculine/male, but was not assigned this gender at birth. This is one of the major subcategories of the transgender population, along with transfeminine and a number of less familiar terms. The term transmasculine is essentially synonymous with the terms trans man and FTM (Female-to-Male), but these terms are more likely to be confused with a transsexual identity by the general public.
The term transmasculine does not tell you anything definite about a person's body or sexuality, only their gender. Even in that area, you cannot predict what aspects of masculine appearance, behaviors, labels, and pastimes any given person might embrace, making the term only a very general indicator of one aspect of a person's personality. However, if someone identifies primarily as transmasculine, it does suggest that they consider their gender identity as a greater issue in their life than their sexuality.
There is some annoyance among members of the transgender population who are FAAB and therefore are automatically assumed to be transmasculine but are actually third-gender, agender, androgyne, or some other flavor. Some also feel that being male does not necessarily mean being masculine. When in doubt, it is generally okay to ask a person their preferred term, rather than sticking with a term you came across on some random website.