Tightening a bolt stretches the bolt and compresses the joined members. The resulting tensile force already present in the bolt before external service loads are applied is the preload.

The bolt as a spring

Within the elastic range, bolt extension and force are related approximately by a stiffness:

$$F_b=k_b\delta_b.$$

The clamped members also behave elastically in compression.

What preload does

Preload creates contact pressure between the joined parts. External separating loads must first reduce that compression before the joint opens.

For shear loading, sufficient clamp force can allow friction between the joined surfaces to carry load without the bolt shank bearing directly against the hole.

Tightening torque is indirect

Assembly torque is commonly used to create preload, but much of the applied torque is consumed by friction in the threads and under the bolt head or nut.

The same torque can therefore produce substantially different preload when friction conditions change.