What opRollup means?

I see a recovered node from a crash with this status but what does it mean?

From the paper https://dgraph.io/paper

To find a balance and avoid gaining deltas indefinitely, we added a rollup mechanism.

Rollups: As keys get read, Dgraph would selectively regenerate the
posting lists which have a minimum number of deltas, or haven't been regenerated
for a while. The regeneration is done by starting from the latest state, then
iterating over the deltas in order and merging them with the state. The final
state is then written back at the latest delta timestamp, replacing the delta
and forming a new state.  All previous deltas and states for that key can then
be discarded to reclaim space.
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Thanks a lot!