In spread betting, DFB stands for daily funded bet.
Spread bets can come in two varieties, DFBs or forward bets.
DFBs run for as long as you choose to keep them open, with a default expiry some way off in the future. The cost of maintaining your DFB position is levied on your account each day: hence daily funded bet. You would generally use a daily funded bet to speculate on short-term market movements.
Forward bets will expire after a set period; instead of paying each day to keep the position open, the entire cost is taken into account in the spread.