As a symbol, the apple usually refers to fertility, love, knowledge, wisdom, and unity.
In Christian mythology, an apple can also be a symbol of sin, or of the original sin (Adam and Eve, that ate the apple). The fact that in Latin the word for apple, mala, also meant 'evil' added to that interpretation. In many medieval paintings one will find a depiction of the divine child Jesus, holding an apple in his hand, as a symbol of Jesus taking the load of all sins upon himself.
In Chinese symbology, an apple is both a symbol of peace and of disease.
"The golden apple represents discord, since its presentation by Paris to Aphrodite in a divine beauty contest, led indirectly to the Trojan War." (David Fontana)
(In Greek Mythology, the goddess Eris, sister of Ares and goddess of discord, threw a golden apple among the gods, saying it was for the most beautiful goddess. Hera, Aphrodite and Athena all claimed the prize. Zeus decided Paris, son of Priamos of Troy, had to decide who would get the trophy. He chose Aphrodite, who then led him to Sparta, where he abducted Helena, which led to the Trojan war.)
(In Dutch the word "twist-appel" - apple of discord - refers to that meaning).
In Norse Mythology, Freya, "the Norse goddess of love and magic, gave apples of immortality from her garden to rejuvenate the gods." (David Fontana, The Secret Language of Symbols). Iduna, another Norse goddess, kept apples that gave eternal youth.