A slicer is the program that takes a 3D model and "translates it" into the sequence of actions that the printer needs to do to create that model. It is called a slicer because 3D printers build the models in horizontal layers, or in other words, in slices.

Cura is one of the slicer programs available. There are many, divided between slicers for FDM printers (the ones that print from a spool of material) and slicers for resin printers (the ones that print from the disgusting goop that comes in bottles). Your printer tends to be packaged with a suggested one but usually you can use any of the appropriate type.

Slicing is one of the most important parts of 3D printing, and it tends to be the difference between ending up with a pristine figure or a very blurry one. In the most extreme cases, good slicing will be the difference between a successful and unsuccessful print.