Let's say, that we have some spin system with multiple sites. This system is in a quantum state denoted by $|\psi \rangle$. When we compute e.g. $\langle S_1^z \rangle = \langle \psi | S_1^z | \psi \rangle$, this can be understood as measuring the expected value of spin in the $Z$ axis on the first site.
But we can also calculate the value $\langle S_1^z S_2^z \rangle$. What is its physical interpretation? Can we compare this to measuring expected value of combined spins at sites 1 and 2 in the $Z$ direction?