The "Altar of Rites" is a special feature that was added to Diablo 3 with patch 2.4.0.
Once you have access to the Altar of Rites, you can unlock sigils by spending materials. There are a total of 26 sigils that offer various abilities and bonuses, including increased resilience, additional damage and reduced damage.
The legendary potion powers in the Altar of Rites are also very useful and can give you massive bonuses, such as resetting all cooldowns or getting all rune effects on a skill.
Overall, the Altar of Rites is an interesting addition to Diablo 3 that gives you the opportunity to further improve your skills and equipment and master new challenges.