Thatch is a collection of dead plant parts like stems and grass clippings that accumulate on the soil surface. That is only a problem if it becomes thicker than .5 inches and grass roots begin to grow into it as if it was soil. That dries very quickly in hot summer conditions and grass roots growing in it are much more susceptible to drought. When mowed at the right height and at the right time, Bermuda grass lawns to not usually accumulate excessive amounts of thatch.
The best way to prevent thatch from accumulating is to begin the year with a clean slate. Before your Bermuda grass begins to green in the spring mow the entire law as described in the "Mowing" section to remove dead top growth. After removing all the debris, rake the entire lawn vigorously with a metal tined leaf rake to dredge up as much that as you can. This takes quite a bit of energy and requires that extra level of motivation. You can also use Lazyman liquid Gold or Lazyman liquid de-thatcher to keep excessive thatch at bay.
Occasionally, hybrid Bermuda grass lawns will accumulate greater than .5 inches of thatch during the growing season. This is usually as a result of improper mowing or excessive fertilizing. In this case begin by aerating (if you haven't done so already). If the problem doesn't go away in several weeks, you may want to rent a vertical mower or "de-thatcher" from a tool rental store. Vertical mowing is tricky though, and should be performed with extreme care. Do not cut too deeply or make more than one pass over the entire lawn, never vertical mow during periods of drought. After vertical mowing, rake and remove all the thatch brought to the soil surface and irrigate.