1. Leviticus 17:3 (ESV)
  2. Exposition

What does this first command refer to?

Leviticus 17:3 (ESV)

3 If any one of the house of Israel kills an ox or a lamb or a goat in the camp, or kills it outside the camp,

No one is allowed to sacrifice a cow, lamb or goat anywhere except at the altar of burnt offering in the courtyard of the tabernacle. This would be a serious profaning of worship. There is only one legitimate altar; any kind of sacrifice anywhere else is improper.