A logarithm is an exponent. The base of the logarithm is what it is an exponent of. The argument of the logarithm function is the value that you get when you raise the base to the power indicated by the exponent.
Here, the logarithm is 4, so that is the exponent of the base, which is 2. The argument of the log function is 16, so that is the result of raising 2 to the power of 4.
In exponential form, the given expression is equivalent to
