A good track usually has a nice clear sound down in those bass frequencies which doesn't sound muffled. Many mixes from beginner producers struggle to get a great sounding mix and complain of a muffled sounding mix in the lower frequencies. Usually the cause is that too many instruments are competing for the space within the bass.

A commonly used technique to leave room in this area is to use some low cutting of EQ or a high pass filter on certain problem instruments. For example, imagine we have a bass guitar and a standard electric guitar. When both are playing together, the electric guitar will still be producing some frequencies down in the range where our bass should sit. Applying a high pass filter (cutting out the low frequencies) on the electric guitar can remove these often unnecessary frequencies that the electric guitar is producing and thus allowing the bass guitar to push through much clearer.