global_step is defined as the number of batches that have been seen by the graph. Each time a batch is provided, the weights are updated in such a direction that it will minimize the loss . Moreover, global_step tracks the number of batches meet so far and when it is passed to the minimize() argument list, it increases by one.
The global_step value can be obtained using tf.train.global_step().
According to your Example:
global_step = tf.Variable(0, name='global_step',trainable=False)
Here, 0 means the initial value of the global_step.