In Ruby, the .is_a? method is used to check if an object is an instance of a particular class or a subclass of that class. It allows you to determine if an object belongs to a certain class in the class hierarchy. The .is_a? method returns true if the object is an instance of the specified class or one of its subclasses, and false otherwise. 

Example:

# Define a class 
class Animal 
end# Create an object 
dog = Animal.new# Check if the object is an instance of a class 
puts dog.is_a?(Animal) # true 
puts dog.is_a?(Object) # true, since all objects are instances of Object 
puts dog.is_a?(String) # false, since a dog is not a String