It is a variant of the focal past, denoting an action in the past which began some time before a time of reference in the past and continued for some time after it, in a progression.
It is used mostly in written language, and is typical of a factual or newspaper style.
● Progressive past, in a discontinuous mode:
Ular yo'lda ketar ekan, turli qushlarning ovozi eshitilib turmoqda edi.
While they were going their way, the voices of all sorts of birds were heard.
O'quvchi bo'lmoqda edilar.
They used to be student. (at that time)