底 (dǐ) means bottom, base, or foundation. It is used both as a standalone noun and as a component in compound words like 海底 (seabed) and 底层 (ground floor). It indicates the lowest part of something.
这个湖的底有很多石头。
Zhège hú de dǐ yǒu hěn duō shítou.
There are many stones at the bottom of this lake.
把箱子放在柜子底吧,上面放不下了。
Bǎ xiāngzi fàng zài guìzi dǐ ba, shàngmiàn fàng bù xià le.
Put the box at the bottom of the cabinet — there's no room on top.
杯子底有一点茶叶,你要不要再泡一杯?
Bēizi dǐ yǒu yīdiǎn cháyè, nǐ yào bù yào zài pào yī bēi?
There are a few tea leaves at the bottom of the cup — do you want to brew another cup?