This means that Solver has found the optimal or “best” solution under the circumstances. The exact meaning depends on whether you are solving a linear, smooth nonlinear, or integer programming problem, as outlined below. This message doesn’t appear for the Evolutionary Solving method.
If you are solving a linear programming problem, the Simplex LP Solving method has found the globally optimal solution: There is no other solution satisfying the constraints that has a better value for the objective. It is possible that there are other solutions with the same objective value, but all such solutions are linear combinations of the current decision variable values.
If you are solving a smooth nonlinear optimization problem with no integer constraints, the GRG Nonlinear Solving method has found a locally optimal solution: There is no other set of values for the decision variables close to the current values and satisfying the constraints that yields a better value for the objective. In general, there may be other sets of values for the variables, far away from the current values, which yield better values for the objective and still satisfy the constraints.
If you are solving a mixed-integer programming problem (any problem with integer constraints), this message means that the Branch & Bound method has found a solution satisfying the constraints (including the integer constraints) with the “best possible” objective value. If the problem is linear and the Simplex LP Solving method is being used, the true integer optimal solution has been found. If the problem is smooth nonlinear, the Branch & Bound process has found the best of the locally optimal solutions found for subproblems by the GRG Nonlinear Solving method.