Belonging to spring, or happening in spring. It is a formal, written word, and in modern English it survives mainly inside a few fixed phrases. The commonest by far is the vernal equinox — the day in late March when day and night are about the same length and spring officially begins in the northern half of the world. You will also meet vernal pools (shallow ponds that fill with rain in spring and dry up later, important for frogs), and in botany, plants described as vernal because they flower in spring. Outside science, poets use it more freely and more loosely: vernal air, vernal green, and by extension a vernal freshness in a person — meaning young, new, full of beginning. That second use is a stretch of the first, and it sounds deliberately literary. In everyday speech nobody says 'vernal weather'; they say 'spring weather'. So: read it as 'spring-', and when you write, use it for the equinox and for scientific contexts, and let plain 'spring' do the rest.