This is how that attack works. You generate a set of small primes called the base primes:
2, 3, 5, 7, 11, ...
This was all pre-computation. After this, to find the discrete logarithm of x, you generate powers of x until it factors in the set of base primes. Once that is found, calculating the logarithm is easy.
This requires a trade-off: the larger the set of base primes, the longer the first step takes, but the faster the second step is.
It's a table specifically of the discrete logs of a large (but tractable) number of small primes. Those discrete logs can be used as the input to a separate stage that can calculate the discrete log of any value in the field.