What is the meaning of ”& Co”

& = and
C = cash
o = 0 / "NO"
A pair of parallel lines with " & CO " in between means:
(1) the cheque is crossed; AND
(2) it can only be paid into a bank account, not to cash.

Do you mean that & Co is a part of the name of the payee, e.g. Chan Tai Man & Co.
That menas Chan Tai Man & Co. is the name of the company. Usually this kind of company is operated by a personnel of a professional occupation like auditors, consultants, professional service providers... etc.