The Wikipedia page of Young's Lattice (https://en.wikipedia.org/wiki/Young%27s_lattice) states that for $p\leq q$ the Mobius function is $\mu(p,q)=\left\{ \begin{array}{ll} (-1)^{|p|-|q|} & \text{if the skew diagram } p/q \text{ is a disconnected union of squares} \\ 0 & \text{otherwise} \end{array} \right.$
Question: How can I prove this?
Bonus: And what does "the skew diagram being a disconnected union of squares" actually mean for $p=(p_1,...,p_m)$ and $q=(q_1,...,q_k)$? I guess we should have something like $|m-k|\leq 1$ and $|p_i-q_i|<1$?