I think hotspots refer to files with a lot of I/O activity going on. In tuning for performance, you don't want many 'hotspots' so to say, and instead would like the I/O to be distributed more evenly among the various datafiles, tablespaces and disks.
To detect the 'Hotspots', you can use the V$FILESTAT view and join it with V$DATAFILE.
Select n.filename, f.phyrds, f.phywrts, f.avgiotim
from v$datafile n, V$filestat f
where n.file# = f.file# ;
there are hot spots in buffer cache as well but the concept is basically the same as datafiles, to check for hot spots in buffer cache you have to check for cache buffer chain latch from v$latch_children then join with x$bh (addr and laddr) and order by touch counts in x$bh