It's the past tense of the verb "aimer" (to love), conjugated in the first person singular ("je") with the direct object pronoun "te" (you).
It implies a love that existed in the past but is no longer present. It could be used to express:
* A past love that ended: "Je t'aimais, mais maintenant, c'est fini." (I loved you, but now it's over.)
* A longing for a lost love: "Je t'aimais tellement, et je t'aimerai toujours." (I loved you so much, and I will always love you.)
* A regret for a missed opportunity: "Je t'aimais, mais j'avais peur de le dire." (I loved you, but I was afraid to say it.)
The context will determine the precise meaning.