That link above is a good one:
Very quickly, almost all basses have a deadspot, typically on the G string between the 5th and 7th fret. A deadspot is caused by the neck 'absorbing' a certain frequency, resulting in the note's fundamental to sustain for a shorter period of time than the rest of the notes on a bass.
The minor deadspots on most instruments are barely noticable (i.e., the fundamental note drop-off is faster than the other notes, but still sustains long enough to not be noticable in most playing situations). An extreme deadspot results in the fundamental dropping so quickly that you barely hear that note at all... hence, that one fretted note area on the neck is 'dead'.
Many designs have been proposed to eliminate deadspots (multilaminate necks, graphite bars, different scale lengths, etc.) None of these seem to have much impact (i.e., I've never noticed any difference in the design of a wooden neck bass and the incidence of dead spots). The only thing that seems to virtually eliminate deadspots is a graphite composite neck. I'd much rather play a bass with a deadspot than play one of those.... too much cost in tone and feel and response for me personally!
If possible, it is always a good idea to play long whole notes from the 2nd to 10th fret on the G string on any bass you are considering purchasing, to make sure that the dead spot(s)... which WILL be there.... are not severe enough to bother you. As long as you get a good, quick, full fundamental on each note, and the fundamental does not drop away too quickly, you should be fine.
