The Implantation Dip is a small drop in temperature around the expected time of implantation (usually 7 - 10 days).
While a drop in temp around this time doesn't necessarily mean that you ARE pregnant, if you don't see this dip it doesn't mean that you aren't either. Just statistically it seems that more pregnant charts than non pregnant charts show this drop.
It is thought to occur due to a second estrogen surge (estrogen has a lowering effect on temperatures) around the middle of the luteal phase. After implantation the corpus luteum then starts producing progesterone which then increases temps again showing a distict dip and rise pattern.