It represents pages being read from disk before they are requested.
When SQL Server thinks data or index pages are being read in a sequential manner, it will pre-fetch the pages into the Buffer Cache before they are requested by the user. This is known as Read Ahead processing. Read Ahead can improve the performance of queries that have to process a lot of table or index pages sequentially, by having the pages already in the buffer cache before they are required.
In some circumstances, a high level of Read Ahead activity can be an indication of poorly coded SQL or inadequate indexes. Read Ahead processing is triggered by table scans or index range scans. In some cases defining more suitable indexes or tuning SQL statements can get SQL Server to use more efficient access methods (e.g. direct index accesses) instead of these scans.