What is _a_? and how it works?

[JustinWalker]

public class Dimensions {
    
    private int _a_, _b_;
    
    public Dimensions(int a, int b) { 
    	setA(a); 
    	setB(b); 
    }
    
    public void setA(int a) { 
    	assert x>0; 
    	_a_ = a; 
    }
    
    public void setB(int b) { 
    	assert y>0;
    	_b_ = b; 
    }
    
    public int getA() { 
    	return _a_; 
    }
    
    public int getB() { 
    	return _b_; 
    }
}

Hi everyone.
I am new in Java programming and i need to know what is _x_ and _y_, and the relationship in the setA or setB function, between the _a_ = a.

Thanks