This is a pretty technical dive into Schnorr signatures and batch validation. I had a very fuzzy idea of the concept before reading it. Now, I'm maybe a little less fuzzy, but I think I got the broad scale idea -- and it's cool!
As far as your node is concerned, validating signatures is hard work. Schnorr signatures let your node take a bit of a shortcut when validating the multiple signatures on a transaction.
Instead of looking at each public key and checking that the private key matches, as in ECDSA signatures, Schnorr pubkeys can be added together and checked against the sum of the private keys.
For blocks that only have transactions with Schnorr signatures,
batch verification could reduce block validation time by approximately 16% for fully Schnorr-signed blocks when using two script verification threads.
When it comes to IBD, the gains are less dramatic,
This means that batch verification could reduce IBD time by 2.88%. The improvement becomes more significant as Schnorr signatures replace ECDSA over time.
This is my non mathematical understanding of the idea, at least. Hopefully, some of the more math-minded stackers will weigh in if I'm off-track.