The Beta distribution has two parameters, α and β:
P^(α-1) * (1-P)^(β-1) / B(α, β)
Looking around online, I see it has something to do with degrees of freedom. There seems to be [an accepted answer to this on Stack Exchange](https://stats.stackexchange.com/q/262956/206342) but I was not able to understand that answer.
So, ELI 5 why we subtract one from the parameters.