What does the ' (apostrophe) indicate in the beginning of a cell?

Hello,
I noticed that some of the cells in my spreadsheet have an ’ apostrophe character before the values I have explicitly written for the cell. (The ’ apostrophe is only seen inside the formula bar at the top)

What is the purpose of this character and what circumstances produce it?

Here is an image of what I am referring to:
https://imgur.com/uZ54qh9

Thank you!

(Edit: activated screenshot -AK)

The apostrophe marks the cell as Text, this means even if it looks like a number or a date, it isn’t.

The way I have removed them since a long time is below. There are other ways of course. Cheers, Al

  1. Select the column in which the
    digits are found in text format. Set
    the cell format in that column as
    “Number”.

  2. Choose Edit - Find & Replace

  3. In the Search for box, enter ^.

  4. In the Replace with box, enter &

  5. Check Regular expressions

  6. Check Current selection only

  7. Click Replace All

Thank you!

See also this FAQ.