xis a Keras tensor.
A "Keras tensor" is a tensor that was returned by a Keras layer
A candidate tensor.
A logical: Whether the argument is a Keras tensor.
This function is part of a set of Keras backend functions that enable lower level access to the core operations of the backend tensor engine (e.g. TensorFlow, CNTK, Theano, etc.).
You can see a list of all available backend functions here: https://keras.rstudio.com/articles/backend.html#backend-functions.