An accretion disc is a disc-shaped rotating mass formed by gravitational attraction.
This explanation doesn't seem like much, but it makes more sense when you put it in the context of our universe. Earth formed through the process of accretion; a bunch of dust began to clump together and eventually attracted bigger rocks which clumped until you have the large spheroid that is Earth. However, all this matter didn't just fly together. Gravity pulled particles to what would become Earth over time, and as these particles flew in towards Earth, they often fell into orbit. This orbital pattern of accreting matter rotates a lot like a hurricane viewed from the top. This rotating disc of matter is the accretion disc.