What does tightness mean in optimization problems?

Question

My optimization problem is convex and underdetermined so that there are 2N equations with 2N+2 unknowns. But I tried to compensate for the lack of the number of equations by introducing 2N inequality constraints. Only two unknowns out of 2N+2 are essential to be specified i.e. the goal is to determine the two unknowns. To attain this goal, 2N unknowns of which construct 2N constraints must be calculated. One approach is to relax these 2N parameters to be equal and construct an over-determined problem, leading to increasing the error of the two major unknown values. Then I decided to apply an alternative approach. So, my questions are 1) What is the tightness of the optimization problem?, 2) Is my problem tight?, and 3) What is the best approach to solving these types of problems (GTRS, heuristic, ...)?