Answer: Clothing means attaining. I clothe on you, i.e., on your reason and your actions. It means that you attain them.
The upper one descends to the lower in order to clothe upon him. And then, when the lower holds on to him, the upper one begins to raise him to a higher level in order to help him understand it.
The lower feels that the upper gives birth to him, gives him everything, and takes care of him. And since the lower empties himself and procreates the next Partzufim, he wants to attain the upper: to know who and what he is and what is the purpose of his existence.
After all, now he feels the same need to take care of his own lower Partzufim. In order to fill them, he appeals to the upper one, because one can receive fulfillment only from the world of Infinity.
It turns out that he is obliged to communicate with the upper. Communication with the upper and his attainment is called “clothing.”
For one thing, the lower has the need to give to the one who is beneath him. For another thing, such need and lack of answers leads him to connection with the upper. For yet another thing, he is prepared to attain the upper, because he and the upper are alike as two drops of water.
Such clothing of Partzufim is always completely identical. The lower who clothes on the upper, sees in the upper the same model he has inside. He sees in the upper his own structure, like his own reflection in the mirror. He, therefore, has no problem attaining the upper. That’s why clothing means attainment. The lower sees himself exactly as the upper one wants him to be.
From the 2nd part of the Daily Kabbalah Lesson 12/10/10, The Zohar