A model whose weights stay private, so you usually use it through a service.
A closed model is like a burger place with secret sauce. You can buy lunch, but the cook will not show the recipe.
It helps you use a strong model fast. But checking it, running it yourself, and changing it are hard.
Open-source-model
A closed model is the opposite of an open-source model.
Open weights
A closed model usually keeps its weights private.
API
Many closed models offer their skills through an API.
MaaS
MaaS packages a closed model as a paid service.