Mortgage loans can be assumable or non assumable. If your loan is assumable the person who buys your house may apply to assume your loan and take it over. Often they will not have to pay the origination fee and some other fees and they will probably be able to keep the same rate that you are paying.
If your Mortgage is non assumable then when you sell your house the buyer must arrange a new loan on your house. If interest rates are the same or lower than when you purchased then the loan being assumable does not matter much.
However if Mortgage rates are just a couple of points higher than when you bought your house it can be a great selling point to have an assumable Mortgage loan. It can make several hundred dollars of difference in the buyers payment. This can allow you to get a higher value for your house.