The number one; the basic numeral indicating a single unit or the first in order.
一を聞いて十を知るという言い回しがある。
There is an expression meaning 'to hear one and know ten' (to infer much from little).
りんごを一つください。
Please give me one apple.
まず一歩踏み出すことが大事だ。
It's important to take the first step.