They mean if you insert a tab after some other text.

Word processors and desktop publishing apps tend to have tab stops (sometimes visible in a ruler at the top of the page) and pressing tab goes to the next tab stop. They're about an inch apart (assuming letter or A4 paper) by default, and you can usually also add your own tab stops. For example, you might have text like this:

Hi
Hello

Assuming the next tab stop is to the right of both words, pressing tab at the end of each one would actually bring you to the same indentation level:

Hi      |
Hello   |

Text editors and IDEs don't do that, and instead make all tabs the same size regardless of where they are.

Some people want the word processor implementation in text editors though. The comment you replied to is saying that's dumb, and I agree with them.