Cool, I didn't know this was possible. What is going on here? Why??
a traditional bitcoin address has only one private key to sign transactions. Multisig bitcoin addresses requires signing of transactions with one or more addresses. A company could own an address where multiple addresses are required to spend the transactions.
In this example, it appears that out of a group of three persons, two must sign the transactions. Normally that means if you have 3 persons: Derek, David and Dorian then 2 persons must sign it. But in this example, the two signing addresses are also multisig addresses.
In pseudo code:
traditional two-of-three:
signature = multisig(a,b,c)
this form:
signature = multisig( multisig(a1,b1) , multisig(b1, b2) , multisig(c1, c2) )
This can become very confusing when observing it. Especially if we have a multisig in a multisig in a multisig of which some are 2-3, others 1-2 and so on.