Lambda is used to transform the input data using an expression or function. For example, if Lambda with expression lambda x: x ** 2 is applied to a layer, then its input data will be squared before processing.
RepeatVector has four arguments and it is as follows −
keras.layers.Lambda(function, output_shape = None, mask = None, arguments = None)
function represent the lambda function.
output_shape represent the shape of the transformed input.
mask represent the mask to be applied, if any.
arguments represent the optional argument for the lamda function as dictionary.