In usual business, Oracle reads data from data files into the buffer cache of SGA. A Read request is served from there to the client. However, A direct path read is where the data is read directly from the data files into the PGA rather than into the buffer cache in the SGA.
Direct Path reads have been around for a while when doing parallel operations, however, Serial direct path reads are new in Oracle 11gR2. The direct path read limits the effect of a table that can be on the buffer cache by reading directly into PGA memory.
The direct path read is available only when the Oracle optimizer chooses a full table scan.