In microsoft word, when you highlight existing text you want to replace, you are in

Overtype or Typeover mode.

Explanation:

Overtype/ Typeover mode in Microsoft word is an editing mode in which everything you type replaces something else in your document.

Simply put, When overtype mode is active and you type a letter, all previous text is eliminated and the letter is placed to the right of the insertion point.

When overtype mode is not active, your text is inserted where the insertion point is located and this is known as the Insert mode.