Bottom line
If you are in the Ready Reserve, you and your dependents get unlimited exchange and commissary privileges, the same as a Marine on active duty. Bring a proper ID card every time. Other base recreation facilities (gyms, golf, clubs, marinas, and the like) are open to you on an availability basis. Standby Reserve Marines get a narrower benefit: they can buy required uniform items.
What it means for you
Day to day, the part that matters most is simple. If you are SELRES, IMA, IRR, or otherwise in the Ready Reserve, you can shop the exchange and commissary like any active-duty Marine, and so can your eligible dependents, with no usage cap. The catch is the ID card: no card, no access. Everything else on base (Marine Corps Community Services (MCCS) morale, welfare, and recreation facilities) is available to you, but on an availability basis rather than as a guaranteed entitlement.