It takes as input a list of tensors, all of the same shape, and returns a single tensor (also of the same shape).
layer_minimum(inputs, ...)A tensor, the element-wise maximum of the inputs. If inputs is
missing, a keras layer instance is returned.
A input tensor, or list of input tensors. Can be missing.
Unnamed args are treated as additional inputs. Named arguments are passed on as standard layer arguments.
https://www.tensorflow.org/api_docs/python/tf/keras/layers/minimum
https://www.tensorflow.org/api_docs/python/tf/keras/layers/Minimum
Other merge layers: 
layer_average(),
layer_concatenate(),
layer_dot(),
layer_maximum(),
layer_multiply(),
layer_subtract()