Ligandbook is a public repository for force field parameters for small drug-like molecules and known ligands of proteins.
If you want to simulate a biomolecule with its ligand or the binding of a drug to its receptor, then you probably already have the parameters for the protein or the nucleic acid. But you might still be missing the parameters for the small molecule because there are so many chemically different molecules and parametrizing each molecule at a a high quality can be an art. Have a look if your molecule of interest has been deposited here: Ligandbook currently contains 2933 parameter sets.
The easiest approach is to do a text search: Type the name of the molecule into the search box in the top left corner and browse the results.
For very accurate search results you can carry out a structure search: a molecular drawer will open and you can draw your molecule or a part of it. The results contain all parameter sets for molecules that contain the drawn structure. (This is very helpful if you want to obtain a parameter set of a related molecule as a basis for your own parametrization.)