Minimum Descent Altitude (MDA) is a specified altitude or height in a 2D instrument approach operation or circling approach operation below which descent must not be made without the required visual reference.
A pilot can not go any lower than MDA until they see the runway. If they do not spot the runway then they will then execute a missed approach to the missed approach point (MAP).
Once you see the runway and confirmed everything you may continue your descent and land safely, if not you fly to the MAP and execute a missed approach and re-try the approach. Sometimes with very low visibility you will have to fly quite close to the runway in order to spot it and continue safely.