Sorry for the confusing part.
For example, for a boolean function, there are 4 inputs ABCD, one output K;
if for both the input patterns 1000 and 1001, the output is 1.
Then we say we 'dont care' the D value. Thus, we combine the above two input as a single input: 1003;
Here 3 represent the "dont care" bit.

And similarly we can combine the inputs with "don't care" bit:
if for both the input pattern 1003 and 0003, the output is 1. In this case, only the first bit A is different.
Then we can combine them together into 3003.