You and me under the lemon tree, this sentence has the same meaning as the meme "I am sour". They both mean that the heart is sour, which is very unpleasant. Lemon is inherently sour, so it is often used to describe me as sour and envious.

This meme is mostly used to describe oneself being envious and feeling sour. It is a metaphor for people’s sour psychology when they feel envious.