about vhdl code for cic interpolation filter

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I have not understood clearly what data_24,std_logic_vector(resize(signed(data_23)),24),resize(signed(data_23b)),24) stands for.especially what does signed(data_23b)),24 stands for .can you explain me properly?

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It is an example of adding two 23 bits data(std_logic_vector type) e.g. data_23 and dta_23b such that result data_24 is 24 bits std_logic_vector.

The statement is made up of resize from 23 to 24 (sign extend by one bit) plus cast from signed to std_logic_vector.

The signed is needed so that the tool understands how to sign extend.