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 cloud service. Cloud apps are applications delivered over the internet and run primarily on cloud infrastructure. They are often built using a mix of compute (servers/containers), storage, databases, identity, and networking services from any provider.