it is a thread conditioner. when a few drops are applied to a spool of thread it soaks into the thread and helps cut down on lint, tangles and fraying. (kind of like using bees wax) biggest difference between using it as apposed to using bees wax or a similar product is...
wax does gum up, to a point, sewer's aide does not, also it is easy to use. does not change the color of the thread or leave any marks on your fabrics. can be used when machine sewing/quilting or hand work. works on any of the threads (except of course nylon/invisible threads and those types.
is fairly inexpensive (compared to bees wax) and a little bottle goes a long way. you only use a few drops at a time.