Logarithm is used in many formulations of artificial intelligence. This function has several interesting properties. Log-likelihood is one such example.
The logarithm is the inverse function to exponentiation. That means the logarithm of a given number x is the exponent to which another fixed number, the base b, must be raised, to produce that number x.
You can read this for more details: https://en.wikipedia.org/wiki/Logarithm