With UI composition, what appears to be a single monolithic user interface is in fact made up of multiple separate components. These components can be served from different backing systems (perhaps separate microservices), and can be managed by different teams.

These components can be changed independently from each other, allowing different teams to work in parallel and push out changes as and when they are ready.

There are a number of different options available here - from browser-included web components, different pages in a traditional website, or server-side templating.

This pattern is explored in detail in my book, Monolith To Microservices.

See more patterns.