A soft cup is a plastic vessel designed for use during your period to collect the blood. Because they can be used for up to 12 hours you may not need to use as many soft cups as you would tampons or sanitary towels. Soft cups are disposable and do cost a lot more than tampons so they can work out quite expensive.
Another use for soft cups is in trying to get pregnant where you use the soft cup to keep the sperm in place after intercourse. Many people have used them with success but it is debatable as to whether or not they would have got pregnant anyway otherwise. Without any proper trials (which is unlikely to happen) the evidence is purely anecdotal, even if some people swear that they soft cup has been the key factor in getting them pregnant.