No, PySpark is not an API, rather PySpark contains Python API to facilitate Big Data processing using Apache Spark. Apache Spark as you might be knowing is an open-source Big Data processing engine, that has topped the performance among all its competitors by its in-memory operations are now matched with a general-purpose high-level Programming Language, Python, which is very easy-to-learn and uses and has extensive libraries, for Data Science and Machine Learning operations.
If you wish to learn PySpark, then check out the PySpark tutorial, and if you are serious about getting certified in it, then check out the PySpark certification course, from Intellipaat. Also, look at the following video on Learn PySpark and get started.