Sess.run with string variable name

Question

I have been studying the book "Tensor flow for Dummies" and there is a piece of code to restore variables earlier stored

with tf.Session() as sess:

# Load stored graph into current graph
saver = tf.train.import_meta_graph('output.meta')

# Restore variables into graph
saver.restore(sess, os.getcwd() + '/output')

# Display value of variable
print('Variable value: ', sess.run('x_result:0'))

Here the variable was defined with the text name as 'x_result' however in the last line we are calling the variable by 'x_result:0' what is the ':0' for. Further when I remove the ':0' I dont get the required result. Please help me understand what all purposes does the ':0' serve