Think back to when you were in school and remember the words PRE and POST. Pre meaning before and Post meaning after. Now just apply that to weed treatments.
When a PRE emergent is applied, it is designed to help control weeds BEFORE they start to grow. It is much easier to impact a weed when it is in its seedling stage than in a mature state of growth. When a POST emergent is applied, it is designed to impact weeds AFTER they have emerged from the seed and this can prove more difficult. PRE emergents should be applied in early spring and again in late summer to help tackle the seasonal changes of winter and summer annuals.