FROST (Flexible Round-Optimized Schnorr Threshold Signatures) is a protocol that minimizes the number of rounds of communication between participants in Schnorr signature schemes, reducing network bandwidth, time, and probability of errors. It requires a maximum of three rounds for signing, even without a trusted signature aggregator or preprocessing stage. It can be used to implement 'n-of-m' threshold signatures represented by a single signature on the blockchain. This saves block space and increases privacy by making them indistinguishable from other, more common spend types.
More information on FROST can be found by reading its official whitepaper and watching the visual explainer below.