Stalling means stop or cause to stop making progress.
If the RLC transmitting window becomes full then the RLC entity may not transmitt any new RLC PDU until the lower edge of the transmitting window is advanced. This condition is termed as Window Stalling.
In specs it is described that what are the actions need to be taken to address this problem ever arises.
Window stalling limits the throughput for lower edge sizes 400 and 1000 bits at the beginning. It can be vary by implementation.