"I am new to microcontrollers"

It's a general programming technique - not restricted to microcontrollers (even Excel has lookup tables!)

In fact, the answer is all in the name; it's just a table (or "array" in programmer speak) in which you look-up a value to find the answer to some question.

I hope you're familiar with the mathematical notation

y = f(x)

which means, the value of y is related to the value of x by some function f
In 'C', we could implement this using a lookup table (array):

y = f[x]

f is an array in which each element contains the value of y corresponding to the index x

One application is in implementing trig functions; eg,

int sin_table[] = 
{
       0, // 1000 * sin(0)
      17, // 1000 * sin(1)
      35, // 1000 * sin(2)
       :
     500, // 1000 * sin(30)
       :
     707, // 1000 * sin(45)
       :
    1000  // 1000 * sin(90)
};

Note the factor of 1000 - to give useful integer values (sometimes known as "fixed point")