DL = Deadline which basically means how close you were to solving the block. To solve a block you need a DL of 240 or less. Basically a higher DL is advantageous to lower hash rate miners due to the fact that they are not carrying out calculations per second as fast, so are more unlikely to get a low DL. So, the higher you make the DL, the more chance slower miners earn shares of the block. With most pools, you only get a reward if the pool finds the particular block you have shares in, whereas with Arionum pools, your previous work done is saved in historic shares on blocks that were not won by the pool. When the pool eventually does find a block, all your previous shares are paid out. However, with Arionum pools, for every block not found, your valid shares in previous blocks decrease by 5%.

I hope that explains it adequately