Usually, I think of a Semi Bluff as a large bet where you have a draw but haven’t made a hand on the turn or flop. You are trying to win the hand right there, but you have a chance to make your hand. A total bluff is where you think your only chance to win the hand is to get your opponent to fold. You don’t necessarily have no equity just the board makes it likely you don’t (ie Pair of 6s with three diamond and a 9, 10, J on the board) but you want your opponent to believe you hit your hand.
Semi-bluffs bring that sweet middle ground, you’re not just tossin’ the dice. Total bluffs, though, can be a high-risk, low-reward game. Better to roll with that semi-bluff hustle and keep that equity alive