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.
This ferrule accepts a single (stranded) wire.
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.
This ferrule accepts two (stranded) wires.
They will be specified as “2 x 0.75mm” size for example.
Here’s the crimping tool used for the ferrules.
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.
Here you can see two different wires sizes being used with the appropriate sized ferrules.
An example on the twin wire ferrules being used to connect several items.