Leased access means that someone else can borrow or "lease" a space that belongs to another person or company. It's like when one kid lets another kid borrow a toy for a little while.
In the case of leased access for things like TV or radio, it means that someone pays money to use part of the time on a station to advertise their own thing - like a toy or a store or a restaurant. This is different from buying commercial time outright because the person using the leased access time doesn't own the space forever. They only get to use it for a certain amount of time.
Overall, leased access is a way for people who can't afford or don't want to buy their own TV or radio time to still advertise to a wider audience.