A fixed cost metric in Optimize is one whose value is determined by a fixed cost associated with each of the unique levels in the optimization scenario that are involved in the solution.
For example, suppose you are looking to choose which of many possible locations to place distribution centers and account for or even minimize your overall total costs. The cost of opening each individual distribution center often includes a fixed cost component (in addition to possible variable costs). Any potentially considered location that was not selected would incur zero fixed cost in this model.
Defining a fixed cost optimization metric in LityxIQ is explained here: https://support.lityxiq.com/773516-Define-Optimization-Metrics.