What are they?
They are simply pegged to a non-crypto currency.
Unfortunately most are pegged to the USA dollar, given that is currently the global currency.
The issue is what if the US$ goes south, in a big way?
Perhaps gold would be more stable? But given the current stock market crash, gold is skywards bound, so maybe that is not a great idea either.
But as with all investing known the products & markets is critical.
https://www.ccn.com/crypto-industry-is-betting-big-on-the-future-of-stablecoins/