Base bleed is a method of extending the range of artillery (or naval gunfire) shells. The low-pressure area immediately behind the shell is a primary source of drag during the shell's flight. A base bleed projectile's casing has been designed to direct airflow smoothly into this low-pressure area, usually through slots and grooves designed to redirect the airflow without turbulence. This increased airflow into the low-pressure area lowers the pressure differential between the front and back of the shell, allowing it to travel further. This is a purely passive mechanism that does not require any additional pyrotechnics or systems on the shell, which is an advantage; however, the range extension is not as great as can be achieved with base burn or base boost projectiles.
Note that in many cases, the term 'base bleed' is also used for shells which contain a pyrotechnic gas generator, and thus is a synonym for base burn. In fact, in the years since I wrote this writeup originally, I have seen the references to 'base bleed' in the original sense, that of redirecting airflow, drop sharply and the term now seems to be pretty much a synonym for base burn. It's possible that no ammunition using this form of base bleed is in production.