Bifrost is an extension to the LN peer-to-peer protocol, which would allow you to send client-side validation data using the LN itself, make RGB-20 payments over the LN and unable more advanced things in the future: DEX, decentralized storage, DeFi, advanced smart contracts etc.
To emphasise once again, Bifrost is not a part of RGB, it's an additional protocol that makes it possible for mobile RGB wallets to work on LN. One of the features of this protocol can be imagined as an improved version of Watchtowers currently present in LN and not breaking the backwards compatibility.
From the programming standpoint, unlike RGB or client-side validation technologies, Bifrost is much more flexible, meaning that we can start with rolling out a simple version of it (a few methods are already written in code, so we just need to extend it a bit more to cover the use case of sending over the client-side validated data) and then update it with new functionality further down the road, potentially maybe even becoming a part of LN itself.
From user perspective Bifrost is needed to be finalized before the RGB release both as a protocol and as a code that can be put into mobile wallets thus also showcasing how RGB over LN transfers can be used from a mobile device.