No problem at all.
That doesn't happen very often. But what it means is that the appraiser believes your home (based on comps) is worth more than you're paying. Say you put a house under contract for $150,000. The appraiser does the appraisal, and feels it's worth $160,000. That's good. If the appraisal is accurate the, yes, your Realtor is correct. That's instant equity. In that example, an additioanl $10,000.
It's no problem down the road. You're still buying the home for the contract price. Your lender will approve your mortgage. And the tax assessment won't be based (at least not directly) on the appraisal. Rather, it'll probably be based on what you paid--the lower amount.
So, no problem at all. It's good news. Congratulations.
Hope that helps.