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# ;