what is meant by look up table

In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than u

Its similar to Log or trigonometric table that is used in Mathematics , in Computation its becomes very helpful to keep a pre calculated Static program ( In the form of Array ) which by passes expensive Computation related to them . And the use is subjective to your application

this is a formula in excel from which you can generate some row or column according to some particular field