It's that thick black plastic sheeting you lay down to smother weeds. There's also a lighter weight stuff, called Weed Block, that has holes in it to let water through.
Besides the plastic or Weed Block, you can also use thick layers of newspaper, which works very well in smothering weeds between plants.
To use newspaper, it's best to dampen it in a bucket of water, that helps prevent the paper from being blown about by the wind. You need to lay down a good thickness, at least 5-6 sheets thick, then cover the newspaper with mulch, whether it be be bark, straw, or whatever.