Adam Clarke's Bible Commentary Verse 8. Hollow with boards] It seems to have been a kind of frame-work, and to have had nothing solid in the inside, and only covered with the grating at the top. This rendered it more light and portable.
Matthew Henry Commentary
Verses 1-8 - In the court before the tabernacle, where the people attended, was a altar, to which they must bring their sacrifices, and on which their priests must offer them to God. It was of wood overlaid with brass. grate of brass was let into the hollow of the altar, about the middl of which the fire was kept, and the sacrifice burnt. It was made of net-work like a sieve, and hung hollow, that the ashes might fal through. This brazen altar was a type of Christ dying to make atonemen for our sins. The wood had been consumed by the fire from heaven, if it had not been secured by the brass: nor could the human nature of Chris have borne the wrath of God, if it had not been supported by Divin power.
Original Hebrew נבוב5014 לחת3871 תעשׂה6213 אתו853 כאשׁר834 הראה7200 אתך853 בהר2022 כן3651 יעשׂו׃6213