This is probably the easiest Chinese character to write (and remember). One theory, mentioned by Eve Kushner in one of her Radical Notes, says that in 一 “the stroke depicts a ‘single extended finger’”. In any case, the Chinese hand sign for “one” is exactly that: a single extended finger. The index finger, to be precise.
Note, however, that there are other hanzi with the same meaning: 壱, more common in Japan, and 壹, more common in China (also pronounced yī). Ponte Ryuurui writes that these complex characters are “used in legal documents, for the purpose of avoiding mistakes with numbers, and also for preventing document falsifications”.