This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of thin? It's also thin, though it can be slender as well.

細い is used for linear or cylindrical things, like 細い糸 (thin thread) or 細い指 (slender fingers). It also appears in expressions where something is figuratively "thin," like 食が細い (having a small appetite) and 声が細い (having a faint voice).

You know what else is thin and slender? A garden hose (ほそ). Imagine trickling a thin stream of water over some plants with your slender garden hose, all while admiring how slim and svelte it is.