Down by the salley gardens my love and I did meet; (1-2)
The poem immediately begins with a pretty common setting and romantic landscape. Those willow trees help to frame the poem's context of young love perfectly since young lovers tend to see these first experiences as fairytale like, at least at first.
She bid me take love easy, as the leaves grow on the tree; (5-6)
According to the girl, folks need to take love easy and avoid rushing things. Love should be like those leaves growing naturally on those trees, instead of some sort of race to the finish line. We understand here that love is both simple and complicated when it comes to allowing it to grow the way it should.
But I was young and foolish, and now am full of tears. (15-16)
There's that refrain again that tells us how young love usually ends. Usually this happens because kids are just like the speaker and don't take good advice. They end up full of tears after having learned a very important (if painful) lesson on life and love.