Adam Clarke on Exodus 25:2

That they bring me an offering - The offering here mentioned is the תרומה terumah, a kind of free-will offering, consisting of any thing that was necessary for the occasion. It signifies properly any thing that was lifted up, the heave-offering, because in presenting it to God it was lifted up to be laid on his altar; but see Clarke on Exo 29:27 (note). God requires that they should build him a tent, suited in some sort to his dignity and eminence, because he was to act as their king, and to dwell among them; and they were to consider themselves as his subjects, and in this character to bring him presents, which was considered to be the duty of every subject appearing before his prince. See Exo 23:15.