Applications that run on cloud servers instead of entirely on your device. Like using a tool that exists in a workshop you can visit anytime, rather than owning the tool yourself.
Gmail is a cloud app where your emails are stored and managed on Google's servers, accessible from any device with internet.
“Cloud app” is a general concept, not a single vendor service. Cloud apps are typically built and hosted using a mix of compute (e.g., containers or serverless), storage, databases, identity, and networking services from any cloud provider.