On this page of the Elden Ring guide, you will learn where to find all Sites of Grace in Lake of Rot. You will gain access to this region after reaching the south-eastern part of the Ainsel River Main.
Among other things, Sites of Grace serve as checkpoints. If you die, you will respawn at the Site of Grace with which you have most recently interacted with.
Lake of Rot is a very large area. While exploring it, you will get inflicted with scarlet rot. If you want to explore this location, it is worth equipping yourself with items that reduce scarlet rot to zero, and put on armor that protects against it.