`x`

is a Keras tensor.A "Keras tensor" is a tensor that was returned by a Keras layer

`k_is_keras_tensor(x)`

A logical: Whether the argument is a Keras tensor.

- x
A candidate 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://tensorflow.rstudio.com/reference/keras/index.html#backend.