The db file sequential read event signifies that the user process is reading data into the SGA buffer cache and is waiting for a physical I/O call to return. It corresponds to a single-block read.
Single block I/Os are usually the result of using indexes.
On rare occasions full table scan calls could get truncated to a single block call due to extent boundaries, or buffers already present in the buffer cache, if so then these waits would also show up as 'db file sequential read'. Usually when Oracle carries out a full table scan however, it will read multiple blocks of data in sequence, and the wait state assigned to this is 'db file scattered read'... a somewhat confusing naming convention given that many people think of a multi-block read as sequential I/O!
For further information on db file sequential read you can check columns in V$SESSION_WAIT for the following information:
P1 - The absolute file number
P2 - The block being read
P3 - The number of blocks (should be 1)
The Wait State report will display a profile of Oracle wait events over time, and will allow you to quickly spot excessive waits. See Create and Interpret Reports#SQL Wait Report.
AppDynamics for Databases has detailed monitoring and reporting related to Oracle I/O. Complete correlation between SQL and Oracle Data File wait is possible, as well as a comprehensive report on Physical I/O.