You will find these with a variety of names, bra strap convertor, bra strap connector and bra strap holder are the most common. They can be worn in the back or front of the bra to hold the straps together.
They are similar to a racer back clip in use. They can stop straps slipping off your shoulders and pull straps out of the way for clothing where they would normally show. They aren’t as easy to find in stores, you can get them online on websites like Amazon but they aren’t as common as racerback clips. You can also make your own quite easily.
Using it across the front for tops or dresses that come in narrowly in the front is one use for it (see illustration). Using it on a bra that has straps that aren’t too widely set will give a better result.
If you need one and can’t find one or don’t want to make one you can use string or cotton to tie the straps together. Make sure you use something that won’t hurt if it rubs.