This is an explanation :
Here's the context. Friar Lawrence has agreed to marry the couple that day, in the hopes that the marriage will end the feud between the Capulets and Montagues. Romeo is still impatient even after getting what he wants from Friar Lawrence:
ROMEO
O, let us hence; I stand on sudden haste.
FRIAR LAURENCE
Wisely and slow; they stumble that run fast.
(meaning---Don't be in such a hurry; look where you're going, or you'll fall flat on your face.)