Most people don't know their engine code. That usually doesn't affect their lives too much, but when asking questions of a technical nature on a car forum it can be very a useful thing to be able to mention.
Why? It gives a more specific, precise definition of the engine in question than any other description you might otherwise use.
Why is that important?
1. Information in parts catalogues, particularly for anything even slightly engine related, is often different depending on which engine code is being looked for.
2. Information (relating to the engine at least) in factory wiring diagrams is split up into sections by engine code. If you don't know the engine code, there might be a number of different diagrams with significant differences which could lead you astray if you pick the wrong one.
3. Questions concerning diagnosis of a fault should often vary depending on the engine code. If you have a 1.2 litre 3-cylinder petrol engine for example, it might have an EGR valve if it has certain engine codes, and not if it has others. It is very likely to even have a different number of camshafts and valves depending on which code. EGR faults are very rare on cars that don't have one!
4. If you know your engine code, you can put it into ebay as a search term (e.g. "AZQ engine") and you will see listings of ones for sale. These can be very useful for finding things on the engine or working out how to get to them, as they often include good photos from several angles of engines on pallets, with varying numbers of ancillary items fitted. Much easier to see what's what than from more or less any angle while the engine is in the engine bay.
How and where can I find this code?
On cars up until about 2018, there tended to be a paper sticker on the boot floor, often near the spare wheel well. Duplicate could often found at the front of the service book.
That includes a certain amount of info about the vehicle, including engine (and gearbox) codes, paint code, all sorts. Example (engine code in red square, gearbox code in blue):
Many cars also have a barcode sticker on the upper cambelt cover, like this one:
The engine code is the letters (in yellow box) just ahead of the engine number:
See also Nige's reply below about finding the engine code on your V5c. Letters ahead of the engine number.
Consider adding this engine code to any info you may have already put in your profile and or signature, and keep it updated if you change cars.