A disk read I/O where the requester must wait for the disk read operation to complete. In SQL Server, the requesting session waits while the page is read from disk. This is normally the most common type of read operation in SQL Server.
Most modifications to data and index pages are made in the Buffer Cache, and after the change has been recorded in the Log, the user can continue without having to wait for the data and index pages to be written to disk. However, certain operations, such as create index, bulk insert, and restore, can require the user to wait for the data and index writes to complete.