Increasing this value will give greater accuracy to the constraint calculation.
Setting this value to 3 is a good starting point for most use-cases.
When using a Connections constraint-type, greater Iteration values will also create much stiffer springs.
This can be useful if you have the Stiffness setting on max, but still require more tension.
If the simulation becomes too stiff, this can be tempered, either by using the Damping parameter or in the settings for the individual constraint.
nexus_constraints_020[Iterations].mp4
Animation demonstrating the effect of Iterations, set at 1 on the left and 5 on the right.
This parameter enables you to dampen any forces necessary to bring simulations under control, should the Iterations value need to be set higher.
nexus_constraints_030[damping].mp4
In this animation, the increase in the Damping value is affecting the Constraints forces globally.
From the drop-down arrow, you can select the following constraints, each with their own setting parameters: Connections, Collisions, Forces, Viscosity, Friction and Surface Tension.
The settings for each can be found in the individual pages below.