A "base" is a hormone with no ester attached. The various esters (long/heavy to short/light molecularly) will cause the hormone they are attached to, to be longer acting in the body than bases. Our bodies must first cleave off the ester before the hormone can attach to a receptor site. With a base, there is no downtime, the injectable base is ready and willing to attach to the receptor as is.
Bases are commonly prepared as suspensions for IM injection, such as testosterone base (which coincientally is most commonly referred to as simply "suspension")