Instantiation is the process of creating a new object from a class definition i.e. instantiation is the process of creating something that is instance of a class. Any instance of any class is an object. Another name for an object is an instance of an class. One class can have lots of instances.
This process is also known as creating an object.
This process is also known as realizing a class.
If you have a plan for building an house then getting the materials and hiring a bunch of construction workers is creating an instance of the house, the process is known as instantiation.
Java has a keyword named "new". There is only mechanism in Java for creating objects from a class and it's by using the keyword "new".
You start with the class definition and you end up with an object of that class, that's all instantiation is.
Here's a great explanation on instantiation in Java, Click to read.