‘Coolant’ in this context refers to the solution of antifreeze and water that transfers heat around and out of the engine and its associated ancillary parts. You can buy readymixed types that you can just pour in straight from the bottle, but concentrated fluid tends to be more cost-effective, especially if you maintain more than one vehicle. Most car manufacturers recommend a 33%-50% mix of antifreeze and distilled/deionised water, but check your handbook and workshop manual for the correct concentrations.
While the cooling system appears not have changed very much, the chemistry behind modern coolants has become immensely sophisticated. One of the main reasons for the change was to increase drain intervals, but the downside is that newer coolants can corrode certain metals that were used in older engines. IAT (Inorganic Additive Technology) is the traditional ‘universal’ coolant, common to many pre-1995 engines. Typically, these require draining every other year. OAT (Organic Acid Technology) types possess longer drain intervals – typically every five years – but are incompatible with yellow metals, such as brass or copper. HOAT (Hybrid Organic Acid Technology) antifreeze incorporates both OAT and IAT elements for more recent engines. This aims to address scale formation on hot spots and enhance water pump seal life, as well tackling wear promoted by cavitation. Check your handbook carefully for the recommended coolant type. If only a car maker part number is supplied, consult a credible aftermarket supplier, who should be able to identify a compatible aftermarket alternative. As certain coolants are incompatible with certain engines, be extremely wary of marketing claims from companies that their ‘universal’ product can be used on every vehicle.