A protein on the outer shell, or envelope, of HIV. gp41 is the portion of HIV pierces a helper T-cell's surface protein, CD4, allowing viral entry. The "120" refers to its molecular weight.
HIV enters the cell. It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.