Slicing is how you turn a model you downloaded into a file your 3D printer can print.
When you find things to print, they will usually delivered as
STL files are basically a bunch of triangles that describe how an object looks:
To print these models, they must first be converted into something your 3D printer can understand.
A slicer is a program that converts STL files into GCODE files, which your 3D printer can use.
After it's been converted to a GCODE file, you can generally just upload/copy that file to your 3D printer and it will be able to print it.
What to use
The two main slicers people use are:
Either one will work, however Cura supports more printers but we've had better print results with PrusaSlicer.
If you need help setting up your slicer let us know!