Thermal paste, better known as Thermal grease for the technically correct are two words mentioned quite often within the PC builders community. This thick liquefied solution serves a greater purpose more than anyone would imagine. Without it you would more than likely have an overheated processor.
Thermal Paste or grease is a conductive compound used as an interface that's placed right on the heat sink and heat source, that heat source being the processor. Its main purpose is fairly simple, it eliminates any air gaps or spaces on the processor, maximizing heat transfer to the heat sink.
Image source: Toms Hardware
Unlike most thermal adhesive, thermal paste does not add any mechanical strength to the processor and heat sink, meaning you can separate the two at any given time. Placing the heat sink over the CPU generally have to be fixated in place with screws.
Curious as to what Thermal paste is made of? Well without jotting down the complex chemical compound and formula, its consist of a polymerizable liquid matrix that act as a electrical insulation, yet at the same time being a thermal conductive filler.
Here's a some tutorial fun fact for you, apply too much Thermal paste to your processor and you will have a mess on your hands. Apply just a dab of paste about the size of a chickpea.
Thermal paste isn't expensive at all, probably the least expensive item you'll use when building a computer.