Microsoft Azure is a cloud computing service created by Microsoft. Azure allows its users to many of its cloud services on a pay-as-you-go basis. Azure offers a large number of cloud services such as services related to storage, computing, networking, identity management, machine learning, etc. These services can be used by individual developers to build and deploy applications as well as by enterprises to manage their applications and company infrastructure.
If you are planning on learning Azure, as a beginner it would be better to start with an online course that teaches you the basics and gives you hands-on experience.
You can either take this Azure Certification which will teach you the basics of Azure or if you are just a beginner and wish to get your feet wet you can watch this end-to-end azure tutorial: