HSFG stands for High Strength Friction Grip where a traditional bolt will stop the plates in a connection from moving apart by supporting the weight on the bolt shank in the form of shear stress, an HSFG supports the weight by clamping the plates so tightly together that they can’t slip past each other. The main advantages of HSFG bolted connections are their greater stiffness and their ability to withstand alternating forces. Their behavior under fatigue loading is also better than that of bearing bolted connections. Against these advantages are the costs of HSFG bolted connections.
The bolts are tightened to a shank tension so that the transverse load across the joint is resisted by the friction between the plated rather than the bolt shank’s shear strength. We don’t use HSFG bolts often because they’re expensive and in most cases, they aren’t needed.
In other words, the friction generated between the plates is equal to the force being put on the plate by the weight of the beam it supports.
HSFG bolts are also commonly used in structures that will be exposed to a lot of vibrations, or loadings that will reverse. To make sure that the plates don’t slip past each other, the bolt needs to be carefully preloaded. The area around the bolt needs to be clean and unpainted and the bolt then needs to be tightened to a precise level.
In general,
HSFG  bolts are high-strength structural bolts that have been tightened such as to induce predefined tension in the bolt shank. They also will prevent any slip between the plates, which means your fastener won’t come loose.