慢 = 忄 (heart) + 曼 (long, drawn-out, for the sound). A heart that takes its time — slow, unhurried. (The same 曼 also gives it an old sense of 'lax / disrespectful', as in 怠慢.)
慢 is the opposite of 快: 很慢 (very slow), 慢车 (a slow train). But its warmest use is as a gentle farewell: 慢走 (mànzǒu, 'go slowly') is what a host says to a departing guest — 'take care, don't rush'. 慢慢来 ('take it slow') is comforting advice. Fittingly for an app called 知止, 慢 carries a positive weight in Chinese: slow can mean careful, savoring, unhurried — depth over speed.
A heart (忄) drawn out long (曼) — unhurried, SLOW.
慢走 ('go slowly') is how a host says goodbye — 'take care, no rush'. 慢慢来 ('take it slow') is kind advice. For 知止, 慢 is a virtue: slow means savoring, not lagging.