WSDL or Web Services Description Language is an interface description language that is based on XML. This language is used for describing the functionality offered by a web service. In general, this WSDL is used for a specific WSDL description of a web service and helps outline machine-readable description, of how the service could be called, what parameters it expects, and the data structure it will return, etc. Now, Salesforce offers its own version of WSDL, namely Enterprise WSDL and Partner WSDL.
If you are interested in learning Salesforce and wish to get started with it, enrolling in a good Salesforce certification course will be a good option. Also, if you are a beginner, watch the following video on Salesforce Marketing Cloud, to get started quickly.