Other than the popular BEP-20 and BEP-721 tokens, there is a third token standard that is widely used in BNB Smart Chain. While BEP-20 tokens are fungible and BEP-721 tokens are non-fungible, BEP-1155 is a multi-token standard - the same token contract may include any combination of fungible tokens, non-fungible tokens or semi-fungible tokens.
Fungible tokens work like currency. A $5 bill may be exchanged with another $5 bill without any difference in value. Non-fungible tokens such as Pancake Bunnies are unique. Each bunny is valued differently, has individual traits, and is often treated as a collectible.
A semi-fungible token works the same way as fungible tokens initially. It could be exchanged for another of the same token contract with no difference in value until a particular condition is met. From then on, it becomes non-fungible and cannot be simply exchanged with one another.
An example to think of is that of a ticket to watch Manchester United in a Premier League match. Before the match starts, the ticket could be exchanged for that of another Premier League match (with similar opponent quality) at the same value. After it ends, the same ticket could be of vastly different value depending on its owner. To some, it may represent a historic moment in their life and is extremely valuable as memorabilia. To others, it may be one of many or even represent an occasion to forget. In this way a ticket that was once fungible with others later becomes non-fungible.