Complex sentence: Whereas in a ripple carry adder, each bit of the adder must wait for the "carry" output from less significant bits, in a carry lookahead adder, all the carry outputs are calculated at once by specialized lookahead logic.
Split into simpler sentences: Whereas in a ripple carry adder, each bit of the adder must wait for the "carry" output from less significant bits. However, in a carry lookahead adder, all the carry outputs are calculated at once by specialized lookahead logic.