How does the OverlapCircle Work in Unity?

Question

Below is the Code I read somewhere to keep a check on whether the player is on the Ground or Not.

We have to set the LayerMask whatIsGround from the Inspector.

I know the OverlapCircle method creates some kind of Circle but i don't know what exactly doest it do and what does it check.

How does it use the LayerMask to determine whether it's on ground or not.

public LayerMask whatIsground;
public Transform groundCheck;
public float groundCheckRadius;
public bool isGrounded;

isGrounded = Physics2D.OverlapCircle(groundCheck.position,groundCheckRaadius,whatIsground);