A discrete logarithm scheme is based on a problem in modular arithmetic. It is easy to calculate the result of modulo function, but it is computationally impractical to find the exponent of the generator. In other words, it is extremely difficult to find the input from the result. This is a one-way function.
For example, consider the following equation:
32 mod 10 = 9
Now, given 9, the result of the preceding equation finding 2 which is the exponent of the generator 3 in the preceding question, is extremely hard to determine. This difficult problem is commonly used in the Diffie-Hellman key exchange and digital signature algorithms.
