These questions can be found by googling.
Bass traps are physical objects placed in a room to act as a
low frequency absorbers. Many people complain about too much 'boom'
in their HT room so they place these objects in areas to dampen the effects.
Do a google image search on bass trap to get an idea on the objects used.
Could be panels, foam, cylinders, etc.
Some people solve the problem electrically with an equalizer to tame the
bass peaks.
I saw a show on TV where they just took some cardboard cylinders like
'sonotubes' and fill it with sand and placed two of them in the corners of the
room. To me it was ridiculous because the owner only had two 8" subwoofers
as if that would create lot of bass -- har har