Bismihi ta'ala
According to the mainstream view of our esteemed jurists, the feet is a part of hijab and it is obligatory for them to be covered, like any other part of a woman's body, with the exemption of her face and hands.
However, in regards to Salat, a woman does not need to cover her feet while she is praying, as long as she is in a place where it is not possible for a non-mahram to see her.
And Allah knows best.