Youzhong Li wrote:
What does an apostrophe (') in front of a word mean in an cell
of an Excel spreadsheet?

I assume you are talking about an apostrophe as the first character
in a cell.

It prevents Excel from interpreting the text. For example, I like to
enter "dates" in the form May06. If I do that, Excel treats it as a
date and reformats it in the form May-06. When I enter 'May06,
Excel formats exactly as May06. I do not need the apostrophe if
I have the forethought to format the cell as Text. But usually I don't
have that forethought; and after entering May06 without the
apostrophe, formatting the cell as Text results in a number -- the
way that Excel stores dates under the covers.

How can I change the format with an ' into a text format of
general format?

The usual way works: click on Format Cells Number Text.
The apostrophe remains hidden.