In short that means the Divine light was sent down into the empty space and then there was the breaking of the vessels. Then in the long process of correcting this the light of kindness was brought back up into the vessel of Foundation. [[I forget exactly where this is in the Eitz Chaim, but it is also brought in Rav Nahman from Breslov's book the ליקוטי מוהר''ן [Magnum Opus of Rav Nahman.]
[For a short introduction let me just mention that the basic structure of the Eitz Haim is this> When God wanted to create the world there was no place for it since everything was just God. So he withdrew his presence and made an empty space-a sphere. Then He sent down light [shem beit nun 52] that went down a drop and then went sideways to make an inner sphere of light. Then that went down again a drop and again went sideways to make another inner sphere of light. That is the spheres of Adam Kadmon (the Divine Man) [That Hegel also brings down.] This process continues for the ten spheres of עיגולים round spheres. Then again the process starts with a new beam of Divine light [shem mem he 45.] that instead of going sideways went down vertically and became the form of a man.--that is Yosher of Adam Kadmon. Light then came out from this Divine Man from the extremities but mainly from his forehead nose ears mouth and then eyes. From the eyes the light went down and became עקודים which are mentioned concerning Jacob and the sheep that were striped.(had stripes). When the light went lower you get the breaking of the vessels. the breaking of the vessels means the vessels fell lower into Creation, Formation and the Physical Universe. The light went up but some sparks also fell. So to start the process of correction you have to bring up the vessels and bring down the light. That is what the Ari means when he says the light of Kindness goes into the vessel of Formation in short. The best introduction to this I think is the Eitz Chaim of the Ari [written by Rav Chaim Vital] but David Bronson thinks the Four Hundred Shekels of silver--also of the Ari is a better intro. The edition of the Baal Hasulam [Rav Ashlag]is best.]
[The difference is the Eitz Chaim itself was written directly by Rav Haim Vital. The Four Hundred Shekels of silver is part of the 8 Gates which was put together from the writings of Rav Chaim Vital by his son Rav Shmuel Vital.
[Incidentally to go deeper into the Ari, I found a few people's approaches to be helpful, especially Rav Nahman of Breslov, but also the Reshash (Shalom Sharabi and his grandson), Yaakov Abuchazeira, the Ramchal [Moshe Luzatto] and the Gra's commentaries.]