Simplify the following text for a Grade 14 native English-speaking person.
Original: In the late 1990s , members of IMBAS , a Celtic Reconstructionist organization based in Seattle , began promoting the name Senistrognata , which they say means `` the ancestral customs of the Celtic peoples '' in reconstructed Old Celtic .
Simplified: In the late 1990 's , members of Imbas , a Celtic Reconstructionist group in Seattle , began promoting the name Senistrognata , which they say means `` the ancestral customs of the Celtic peoples '' in reconstructed Old Celtic .