All these instructions sign-extend a short value into a longer one, by replicating the top bit of the original value to fill the extended one.
CBW extends AL into AX by repeating the top bit of
AL in every bit of AH. CWDE extends AX into
EAX. CWD extends AX into DX:AX by repeating
the top bit of AX throughout DX, and CDQ extends
EAX into EDX:EAX.