Metapools allow for one token to seemingly trade with another underlying base pool. This means we could create for example the following pool: [UST, [3Pool]].
In this example users could seamlessly trade UST between the three coins in the 3Pool (DAI/USDC/USDT). This is helpful in multiple ways:
Prevents diluting existing pools
Allows Curve to list less liquid assets
More volume and more trading fees for the DAO
The Metapool in question would take UST and 3Pool LP tokens. This means that liquidity providers of the 3Pool who do not provide liquidity in the GUSD Metapool are shielded from systemic risks from the Metapool.