Acetates are a relic of the days before cassettes, DAT, and recordable CDs came into widespread use in recording studios. Manufactured from aluminum, and coated in a thin sheet of vinyl, they were produced to allow the concerned parties to hear how a particular version of a recording would sound outside the studio, on their home hi-fi, for example.
2002, Dave Thompson, The Music Lover's Guide to Record Collecting, Hal Leonard Corporation , Acetates—The Rock Star's Rough Draft