The lemon is really a called a etrog and is the fruit from a citron tree. It is however, commonly called a lemon or a lemon-like fruit. The etrog is used in the Jewish holiday of Sukkot and is part of the 4 species. The four species are the lulav, the hadass, the aravah, and the etrog and each has a symbolic meaning in Judaism but its easier to understand the symbolism of the lemon if we consider all of the species.
The first explanation is that:
The lulav (date palm fronds) has taste but no smell, symbolizing those who study Torah but do not possess good deeds.
The hadass (leaves froma myrtle tree) has a good smell but no taste, symbolizing those who possess good deeds but do not study Torah.
The aravah (leaves from a willow tree) has neither taste nor smell, symbolizing those who lack both Torah and good deeds.
The etrog has both a good taste and a good smell, symbolizing those who have both Torah and good deeds.
The second explanation is that:
Each of the species represents a body part because their leaves or shape resembles a body part. The meaning is that each part comes together to make mitzvot (good deeds) such as studying Torah as you need posture, good sight, ability to recite, and heart in what you are doing!
Lulav - the spine
Hadass - the eye
Aravah - the mouth
Etrog - the heart
