Short answer: it's like Java EE's baby brother.
Longer answer: After Java EE 8, it seemed like Oracle had basically given up on Enterprise Java. Other frameworks (like Spring) were continuing to innovate and build more cloud-native features, while Java EE stagnated. To correct this, a few companies (IBM, RedHat, Tomitribe, Payara, and probably others that I am forgetting...) got together to create a "Micro" profile (Java EE has the concept of "full" profile and "web" profile) - so the idea was that this would be the minimum set of technologies necessary to build modern cloud-native services. At first it just included EE technologies (JAX-RS, CDI and JSON-B/JSON-P). But then it started creating its own technologies. These include: MP Health, JWT Propagation, Metrics, Fault Tolerance, Config, Rest Client, OpenAPI and Open Tracing. So, much like Java (now Jakarta) EE, when you deploy to a MP-compliant framework (like Open Liberty[1], Wildfly[2], Payara[3], or TomEE[4]) your application can take advantage of these APIs and features.
There are also some standalone MP technologies that are not part of the "profile" but many vendors have chosen to implement - they include: MP GraphQL, LRA (long running actions - basically a cloud-native transaction spec), Context Propagation, Reactive Streams Operators and Reactive Messaging.
In general, MicroProfile works quite well with other Jakarta EE technologies. For example, a colleague and I wrote a sample[5] and presentation[6] on using MP GraphQL with JPA.
If you'd like more information, I'd suggest checking out the MP home page[7] - which has links to presentations about the overall and individual projects, how to use it, what other vendors are involved, how to get involved, etc.
[1]
https://openliberty.io/
[2]
https://www.wildfly.org/
[3]
https://www.payara.fish/solutions/eclipse-microprofile-and-the-payara-platform/
[4]
https://tomee.apache.org/
[5]
https://github.com/OpenLiberty/sample-graphql-jpa
[6]
https://www.youtube.com/watch?v=RzrkjuA3LvU
[7]
https://microprofile.io/