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The Citadel Chaos Black spray isn't a primer, it's a basecoat. You should always prime models with a primer as this will ensure the best possible surface to paint on. Not only will you have a nice uniform colour to start with, you'll also have a slightly rough and porous texture (called a 'key') which will help your paints stick better. Paint directly onto bare metal or plastic tends not to cover well as the oils from the moulding process or your fingers will form a barrier on the surface of the model which repels the paint and makes it go streaky.
It's a little confusing as some people use the terms interchangeably. They aren't always the same things however.
A primer primes the surface for painting. It should be the first step because it adheres well to bare metal or plastic and gives a finish that is slightly porous and has a little bit of texture to it. These properties mean that paint applied on top of it will cover better. The Skull White spray is a primer.
An undercoat is a coat of paint that provides a good colour to start painting from. For most people, this will be the primer they use as well but it's possible to prime with one colour and then undercoat with a different one afterwards - for example I prime my Grey Knight vehicles with the white primer and then undercoat them with Chainmail from my airbrush. Chaos Black spray is an undercoat.
A basecoat is often used to mean exactly the same thing as an undercoat. When it's used to mean something different, it means a base colour for a specific area of colour that you then shade or tint in some way. For example I undercoat my Grey Knights with Chainmail but then I use Scab Red as a basecoat for the weapon housings.