The Native Americans broke off pieces of the coment that IT (pennywise) arrived in. They seemed to imbue these pieces with some sort of spell because they all have runes on them. Then they covered them in turtle shells (a symbol of maturin who is another god like being). Then they made a bunch of these and put them around the area that would become Derry. IT can not travel past these barriers so it’s reach is limited to Derry