一 is the simplest character in all of Chinese — a single horizontal stroke. It looks exactly like what it means: one. In oracle bone script over 3,000 years ago, it was already just one line. Two lines became 二 (two), three became 三 (three). Counting couldn't be simpler.
In Chinese philosophy, 一 is the origin of everything: 道生一,一生二,二生三,三生万物 — 'The Way gives birth to One, One to Two, Two to Three, and Three to all things.' One isn't just a number; it's wholeness and unity. 一 also changes tone like 不: it's yī alone, but yì or yí depending on what follows.
One single stroke = the number ONE. The easiest character you'll ever learn.
一 changes tone depending on the next word: yì before 1st/2nd/3rd tones (一天 yì tiān), yí before 4th tone (一个 yí gè). Only 一 and 不 play this tone-changing trick.