Fashion tape is double sided tape which you use to keep clothing in place. The main difference between fashion tape and normal double sided tape is that the adhesive is one that is meant to be used with skin. A lot will still recommend doing a patch test before using it to make sure you don’t react. It also shouldn’t damage your clothes. If you are worried about this you can do a test on the inside of a seam before using it, although generally you will only be using the tape on the underside of the fabric anyway.
Common uses are to keep loose straps in place and on plunging necklines you use short pieces of it under the edge to stop it falling open when you move. You don’t have to stick fabric to skin, it can be fabric to fabric as well. If you have a bra that is likely to be seen as clothing shifts you can tape the clothing to the bra. If you are wearing a button shirt and it’s gaping open you can snip a small piece of fashion tape off and use it to secure between the buttons.
Note: The woman in the picture has put the tape on her skin first, it’s usually easier to attach it to the clothing first.
You can get fashion tape in lingerie departments as well as online on websites like Amazon. It’s available in short strips or on rolls. It’s a good idea to carry some with you in case it comes undone and you have to reattach it. So look out for ones that come in a small container, or put a few spare strips in your purse.