It takes as input a list of tensors, all of the same shape expect for the concatenation axis, and returns a single tensor, the concatenation of all inputs.
layer_concatenate(inputs, axis = -1, ...)A tensor, the concatenation of the inputs alongside axis axis. If
inputs is missing, a keras layer instance is returned.
A list of input tensors (at least 2). Can be missing.
Concatenation axis.
Standard layer arguments (must be named).
https://www.tensorflow.org/api_docs/python/tf/keras/layers/concatenate
https://www.tensorflow.org/api_docs/python/tf/keras/layers/Concatenate
Other merge layers: 
layer_average(),
layer_dot(),
layer_maximum(),
layer_minimum(),
layer_multiply(),
layer_subtract()