This is a special foot for your overlocker which is used for sewing tapes on stretch materials (knitwear) the seams are prevented from stretching when a tape is sewn on 

Use for inserting a tape into seams that stretch easily and require extra stability. 

Suitable for MERRYLOCK overlock machines. 

FOR SINGER OVERLOCK QUANTUMLOCK SERGER OVELOCKER 
DESIGNED FOR SINGER QUANTUMLOCK SERGERS-MODEL 14T967,14T957DC AND 14T948DS 

Comes with instructions on how to use the attachment.