Literally: one – garment – belt – water
Alternately: A relatively thin strip of water, such as a river or narrow stretch of sea. By extension, two things that are very close together.
Notes: Unusually among four-character compounds, many of which can be broken into a pair of two-character terms, this one is structured 1-2-1: It describes a single (一) sash-like band (衣帯) of water (水). It comes to us from the History of the Southern Dynasties (南史, in Japanese Nanshi).
Several variants and synonyms of this phrase exist, including 一牛鳴地 (ichi gyuu mei chi), which is a distance close enough that you can hear a cow across it.