A foot printed on the ground — three toes above, the heel below. It first meant the foot itself and by extension going, then flipped to mean the opposite: where the foot comes down, it stays. Every character about walking still stands on it, 步 (a step) being two of these footprints one after the other.
止 is half of this app's name. 知止 — 'know where to stop' — opens a line in 《大学》: 知止而后有定, knowing where to stop brings stability. 停止 is to cease, 禁止 is forbidden, and 止步 on a sign means go no further.
A FOOTPRINT pressed into the ground — the foot has come down, and it STOPS there.
止 is the 止 in 知止, the name of this app. 《大学》 opens with 知止而后有定 — knowing where to stop is what brings stability.