I’m no expert hahaha but I’ll try!
Basically objects when they’re made in blender (or milkshape or anything) are made up of tiny tiny little polygons that join to eventually make up a bigger shape. The more polygons – or generally the more intricate and detailed something is, especially curves etc – the harder it is for your game to process it.
If you have issues with crashing or lagging or anything like that it can mean that your game is having trouble handling high polygon CC items, hence the warning for people who know their computers might not be able to handle it.
Anyone else who knows what they are talking about can feel free to chip in and help explain lol?
High poly items are 1200 polygons per 1 square. This is a general rule, and does vary a bit and is affected by the type of item and how many times it would be used in a room. For instance a statue or feature light can be over that 1200 polys because it would be something not used often, but things such as clutter, which can be used multiple times in one room, can quickly add to lag if they have a lot of polygons.
Generally speaking, it is ideal to keep items around the recommended value, as it takes more processing power to render those items with a higher count. The more vertices (which make up a polygon) means more data for each point, so it requires more memory and resources to show it. That is not to say that higher polycounts are bad, it just means less people will be able to seamlessly use them. Sometimes it is even necessary to get the right look for an object. Curved surfaces are poly-hogs.
Only the person downloading will be able to know if their setup can take high poly items. I always recommend that people doing conversions and new content disclose polycounts, just so the lesser informed people know what they are getting into. You will soon realise if your game doesn’t like this content because you will get frame rate drop, performance issues and tearing. If you run your game on low setting, you should be careful ans these are the games that will have issues with higher poly items because of the age of a graphics card or its rendering capabilities.