"Pending" means that buyer and seller have agreed on the terms of a sale, pending the satisfactory resolution of all contingencies. The most common contingencies are appraisal, financing and inspection. Should any one of those contingencies not be satisfactorily resolved, the sale "fails" and the seller will be open to another offer. Whether the home can be shown, or whether back up offers will be acceptable, in the period between first going pending and closing is up to the seller. If you're interested have you agent check with the listing agent.
I once bought a home that was pending when I first learned of it. The buyer's agent said it was OK for us to see it. We loved it, so I told the buyer's agent to let me know if the first offer fell through. It did, he did, and we bought it. I am not the first nor the last buyer to have that happen.