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If f and g are two arithmetic functions, one defines a new arithmetic function f ∗ g, the Dirichlet convolution of f and g, by
(f*g)(n)&=\sum _{d,\mid ,n}f(d)g\left({\frac {n}{d}}\right)\&=\sum _{ab,=,n}f(a)g(b)\end{aligned}
where the sum extends over all positive divisors d of n, or equivalently over all distinct pairs (a, b) of positive integers whose product is n.
This is from Wikipedia, a bare minimum explanation. See https://en.wikipedia.org/wiki/Dirichlet_convolution for the full explanation.