What did the veil keep the priests from doing?
The veil kept even the priests from any approach to the Holy God, who was represented by the Shekinah glory above the Ark of the Covenant. It barred all but the high priest, who could enter behind the veil only once a year.
Les explains that the veil depicted the very body of Christ Himself. It was a heavy curtain made of beautiful linen, and it kept even the priests from any approach to the Holy God, who was represented in the Shekinah glory, the cloud by day and fire by night. The veil kept any approach from anyone but the high priest once a year. Les notes that the high priest comes behind the veil only once a year, as described in Leviticus 16.