These are from the Bald Cypress tree and are called as Bald Cypress Knees. They are part of the tree’s roots system and specialize in gaseous air exchange, this is because the tree is usually found in wet soils where the roots cannot get air.
Bald Cypress Knees can make your lawn impossible to walk or cut. These knee roots can be cut off without harming the tree and is best done in the winter. Best contact a professional tree service company.