A burner is a phone that you can lose/throw away and not worry about it. It’s meant to be used in public instead of your more expensive phone.
I will typically take a burner with me into destinations that have higher rates of crime. If I lose it, it’s not going to be the end of the world.
This is the phone I use when I’m on the street. If it gets swiped, I’m really not too worried about it.
In the past, I’ve taken two burners with me depending on my destination. You can get a reasonable Android burner for ~$30.
Even though you are willing to lose it, make sure that you secure your phone! When you set your burner up, don’t forget to prepare it for offline use as well.