JVM means Java Virtual Machine. It is a machine that simulates what a real machine does. Like a real machine, it has an instruction set, a virtual computer architecture, and an execution model. It is capable of running code written with this virtual instruction set, pretty much like a real machine that runs machine code.
The hotspot is an implementation of the JVM concept. It was originally developed by Sun, and now it is owned by Oracle. There are other implementations of the JVM specification, like JRockit, IBM J9, etc.
The OpenJDK is a project under which an open-source implementation of HotSpot.
Want to learn Java? Check out the Java certification from Intellipaat.