Wire ferrules (also called bootlace terminals) are crimped onto stranded wires that are being inserted into terminal blocks.
They protect the wire and keep the strands together to avoid strays shorting to adjacent terminals.
You must choose the right sized ferrule for the wire.
They are commonly available in sizes from 0.25mm to 6mm.
For Christmas lights you’d usually be working with the smaller sizes.
For large cables going under power supply screw terminals, you may be better off with regular fork terminals as large ferrules won’t fit under the screw/clamp.
They will be specified as “2 x 0.75mm” size for example.
This one crimps into a hexagon shape, but there are square versions.
You can get the tool from eBay or Altronics, etc.
Don’t bother trying to use pliers to crimp ferrules.