Base paint is the first colour on a primed model: an opaque, heavily pigmented coat that establishes what colour each area is before any shading or highlighting happens. It is not the same thing as primer, which is a preparation coat that exists for adhesion, and confusing the two is a common early mistake with consequences that show up several steps later.
Base, primer and layer, in order
Primer goes on bare plastic, resin or metal and gives paint something to grip. Base paint goes on primer and establishes colour. Layer paint goes on base paint in thinner coats to build highlights. Ranges that use these words as product categories are describing exactly this sequence, and the difference between a base and a layer paint in the same range is pigment density: base paints are formulated to cover in one or two thin coats, layer paints to be built up gradually without obliterating what is underneath.
Why coverage is the property that matters
A base coat's job is to be an even, uninterrupted field of colour, because everything that follows reads against it. Patchy coverage shows through a wash as blotches and through a highlight as inconsistency, and it is much harder to fix later than to do properly now. This is why base paints carry more pigment and why applying them thinned in two coats beats one thick coat: two thin coats cover evenly and preserve detail, one thick coat covers evenly and fills it.
Choosing base colours for an army
The practical decision is which base colours you will use most, because those are the pots you will replace. A coherent force usually rests on two or three base colours plus a metallic, and buying those in the largest available size or as duplicates is more useful than owning a wide spread of colours you use once. It is also worth checking that the base colours you choose have shades and highlight colours available in the same range, because matching them by eye later is harder than it sounds.
Questions people ask about base paint
What is base paint?
The first opaque colour coat on a primed model. It establishes the colour of each area before any shading or highlighting, and is more heavily pigmented than layer paint.
Is base paint the same as primer?
No. Primer prepares the surface so paint grips it; base paint is the first colour. Primer goes on bare material, base paint goes on primer.
How many coats of base paint?
Two thin coats rather than one thick one. Thin coats cover just as evenly and preserve the sculpted detail that a thick coat fills.