In academic publishing, the word backlog usually refers to the phenomenon that a journal has a number of accepted papers waiting to be published for a significant time.
[1] The backlog is usually described in months; for instance, a certain journal could have a backlog of 12 months, which means that on average it takes one year for the journal to publish a paper that has been accepted.
This waiting time adds on top of the time between initial submission and acceptance of a paper, which can also vary.
The American Mathematical Society publishes a list of the backlog of mathematical journals each year in the November issue of the Notices.
[2] If a journal has a heavy backlog, this might have the effect that the journal is less likely to accept papers in the future, or even reject all manuscript submissions temporarily.