MACS stands for Malaysia Automated Clearance System (MACS). Malaysia Automated Clearance System (MACS) has been developed to cater for fast-track immigration clearance for non-Malaysian investors, business persons and professionals.
It is basically like the automated clearance system that you use at the airport (if you're a Singaporean Citizen, Singaporean Permanent Residents or a Singaporean Employment Pass holder). The only downside as I have seen it so far, it is only available in Iskandar (Johor Bahru, Johor) Malaysia.
I have been using this for the past couple of years and applied on the Malaysian side of immigration (the immigration at the causeway crossing at Woodlands-Johor). It is called, "MACS One Stop Center", so if you ask around on "where to get MACS", they will show you.
Note - this just fast tracks immigration but doesn't replace Visa requirements:
MACS is simply a form of fast-track facilitation and does not act as an immigration pass or visa. Visitors are still required to obtain visa (if applicable) and will be issued with relevant immigration pass. Singaporean Frequent Travellers are still required to obtain the relevant long term immigration pass.