Metformin???

So my doc wants me to lose 40 pounds in 3 months then put me on metformin. He said something about pcos insulin resistance but the rest of the convo was about my weight. Did blood test glucose was fine and insulin was borderline high... They never really explained anything just tole me to start a diet and try a brisk walk and lose 40 pounds then that was that... Could someone please explain to me what pcos insulin resistance is and what metformin is supposed to do to help me get pregnant?

Edit* my periods have become irregular for the past year thats why I went to the doctor didnt think I would have to have month long periods for 3 more months before getting help...