That is just the definition of Logarithm which is what is taught to all students today i.e.

Given a^x = b we define log_a(b) = x where 'a' is a +ve real number - https://en.wikipedia.org/wiki/Logarithm#Definition

The above wikipedia page also details the properties, applications and generalization of the logarithm concept which are non-trivial.

As i pointed out above, that does not help in intuiting why it is helpful and needed. That is why you need to read the history of logarithms and see how we arrived at the above standard.

Napier actually calculated logarithms of sines for every minute from 0-90degrees to simplify astronomical calculations. The complexity/sizes involved, precision needed etc. can all be seen in this detailed paper walking you through the entire process of table construction; Napier’s ideal construction of the logarithms (pdf) - https://locomat.loria.fr/napier/napier1619construction.pdf