ABSD stands for Additional Buyer's Stamp Duty.
"Liable buyers are required to pay ABSD on top of the existing Buyer’s Stamp Duty (BSD). ABSD and BSD are computed on the purchase price as stated in the dutiable document or the market value of the property (whichever is the higher amount)."
The excerpt above is from IRAS website here, https://www.iras.gov.sg/irashome/Other-Taxes/Stamp-Duty-for-Property/Working-out-your-Stamp-Duty/Buying-or-Acquiring-Property/What-is-the-Duty-that-I-Need-to-Pay-as-a-Buyer-or-Transferee-of-Residential-Property/Additional-Buyer-s-Stamp-Duty--ABSD-/
You can also refer to the infographic below as a reference to determine your ABSD liability.
3 key criterias applies: 1. Residency Status, 2. Property Ownership Count. 3. Individual or Entity.
Infographic from ERA Realty Network Pte Ltd
Disclaimer: Please seek advice from the relevant authorities if you are unsure. The information above is meant as a reference.