body_entered signal will be generated by the Area2D node whenever a PhysicsBody2D node enters the collision shape of the Area2D node. You will need to connect the Area2D node's
body_entered signal to the function that you want it to trigger. The triggered function will be passed an argument (the PhysicsBody2D node that entered the area). So make sure the triggered function has a single parameter (see code snippet).
# Put here the code you want to run whenever a body enters the area.
# Remember that 'body' will be the PhysicsNode2D that entered the area.
print(body.name + " entered the area")
print("velocity " + str(body.linear_velocity)) # assumes body is RigidBody2D, since KinematicBody2D and StaticBody2D nodes don't have a linear_velocity property
Remember that I had to connect the Area2D node's
body_entered signal to this function, and that this function could be in the Area2D node or it could be in any other node. For information about connecting signals to functions I recommend the signals tutorial contained in the docs. Also, feel free to ask additional questions.