It's the number of work items that are still unfinished since the last time work was obtained. If this number reaches zero, the SPU/PPU miners will be starved for work. On the other hand, there is little point in keeping a huge backlog, so the work items are trimmed automatically to keep the number reasonable.

The number is essentially just informational. The number will drop each time a new block is being worked on, since the backlog is then essentially useless and has to be cleared. The only time to be concerned (in terms of overall efficiency) is if you have a long period of RPC errors during which the entire backlog is depleted.