What is meaning of placeholder variable?

Answer 1

A placeholder is used for feeding external data into a Tensorflow computation (stuff outside the graph). Here's some documentation: (https://www.tensorflow.org/versions/r0.10/how_tos/reading_data/#feeding)

TensorFlow's feed mechanism lets you inject data into any Tensor in a computation graph. A python computation can thus feed data directly into the graph.
https://stackoverflow.com/questions/41294094/is-there-a-difference-between-a-placeholder-and-variable-when-not-building-a-mod/41294283

Answer 2

x=('{} man'.format('Hi'))
#output=Hi man
In this code,(curly brackets ){} in x variable is placeholder and Hi is placeholder variable.This is a string formatting concept.