Spiritual beings in physical bodies

"that you shall take of the first of all the fruit of the ground, which you will bring from your land, which the Lord, your God, is giving you. And you shall put [them] into a basket" [Ki Tavo 26:2]

First fruits represent the Jewish souls, as the Midrash teaches that the Divine Thought to create Jewish souls "preceded everything" [Bereishis Rabah 1:4]. In this respect, Jewish souls are "first" and cherished in a way similar to first fruits.

When the soul is in Heaven, before it enters a body, it enjoys an intense, ecstatic relationship with G-d.  Nevertheless, the soul is sent down to earth in a body which conceals its relationship with G-d.  This is not without profit since, through this descent, the soul is able to carry out a mission in the physical world, which can eventually result in an even more intense relationship with G-d.  For G-d's innermost "desire" is for His mitzvot to be carried out specifically in the physical world.

This is the lesson from the requirement of first fruits being placed in a basket.  Although the fruits are the finest and first of the crop, they cannot achieve perfection without a basket. 

Likewise, the Jewish soul, which is G-d's highest priority, cannot achieve perfection without coming down into a physical body, in a world of concealment and temptation, because it is precisely through that descent that an even greater ascent is achieved.

Source: Likutei Sichos Lubavitcher Rebbe vol 29

Also see: Yerida L'tzorich Aliya: Descent for the Purpose of Ascent