An accretion disk is a disk
which forms around an object to which it is gravitationally attracted. The reason the matter forms a disk is that its angular momentum
prevents it from falling directly onto the object. The disk experiences internal forces amongst its constituents which heat up the disk and aid in transferring the angular momentum out of the disk and the matter in onto the object.
This action of the matter falling, or dripping, slowly in is the accretion. It is usually associated with binary stars (one star accreting onto the other), and black holes in the centre of galaxies.