A semi bluff is a situation where you don't have a strong hand at the moment, but you have a good draw to a strong hand and may also win the pot by betting or raising.
For example, your opponent has been betting on each round and on 5th street shows the following hand:
You have:
By raising on 5th street you give yourself two ways to win: you can either get him to lay down his hand now, or you can complete your flush or straight draw to win the hand even if he does call. You may also gain a free card on 6th street if he checks and you have missed your draws.