Hepatitis B is a virus that attacks the liver in humans and travels throught the bloodstream. The primary ways hepatitis B is contracted are through transmission of blood fluids, sexual intercourse, contaminated needles among drug-users, and vertical contact between mother and unborn child. The virus is rarely fatal, but can cause fever, jaundice, vomiting, and lowered functionality of the liver.
The stage you are referring to is often called the latent stage in viral development. When the virus makes it's initial invasion, replication takes place within the host's liver cells. The body's immune system recognizes the viral invaders and starts the process of combatting the virus. Part of the virus is left within the cells of the liver, however, and for lack of a better term, "lies low" while the battle rages within the host's blood stream. The virus thus reemerges after a certain recovery period. People who exhibit this cyclical pattern are said to have "chronic hepatitis B" and are identified as carriers of the virus.