What does it mean? What do I do about it?
You can increase the before-image buffers in the before-image buffer pool with the Before-image Buffers (-bibufs) startup parameter. Increasing the number of buffers increases the availability of empty buffers to client and server processes. In general, initially set this parameter to 20. Increase it if there are any empty buffer waits
See also How to tune BI I/O performance.