Adhesion receptors are attaching the virus in a reversible manner to target cells or organs. This adhesion is not mandatory for virus entry, and alone do not trigger entry. Nonetheless it enhances significantly infectivity by concentrating the virus in the vicinity of it's entry receptors, or bringing it to an organ in which are it's target cells.
In herpesviruses for example, gC makes the primary contact with the host cell surface, binding to heparan sulfate proteoglycans. Binding of gD to the entry receptors induces viral fusion to the host cell membrane and viral penetration 
.