An icon is a bet. The designer bets that a particular visual symbol will be decoded correctly by the target audience, without the aid of a label. Most of the time, this bet is won through convention rather than through the intrinsic clarity of the symbol. The hamburger menu icon — three horizontal lines — means "menu" because millions of apps have trained users to see it that way, not because three lines naturally suggest a navigation menu.
The best icons are concrete, not abstract. A camera icon for photography, a magnifying glass for search, a house for home — these work because they reference familiar physical objects. Abstract icons (geometric shapes used to represent concepts) require more learning and are more culturally specific. When in doubt, pair an icon with a text label; when confidence is high that the icon is universally understood, the label can be removed for visual cleanliness.
Icon sets should have a consistent visual language: matching stroke weight, corner radius, optical balance, and level of detail. An icon set where some icons are filled and others are outlined, where stroke weights vary arbitrarily, and where the level of realism differs between icons will feel fragmented and unfinished. Visual consistency in an icon set is the equivalent of a consistent typographic scale — it signals craft.