下段払い, gedan barai
下段, gedan means low level/downward
払い, barai in this case means to sweep or clear away. By itself it is the root of the verb harau, but in this case the H changes to a B.
Yusuke Nagano is a Shotokan teacher in Tokyo, you can hear how he pronounces it in this video:
One application he neglects to mention is the use of this "block" to literally sweep, as part of a throw.