A title that is too short

Short is good. This is too short to say anything.

Titles need room to work

The title is the most useful string of text you control on a page. A two-character title throws that away. It cannot hold the term people search for, it does not describe the page, and it gives nobody a reason to pick this result over the others next to it.

A good length

Aim for about 30 to 60 characters. That is enough to name the topic, include the words people actually search for, and add a short hook. In practice that is one clear phrase, not a single word and not a full sentence that runs off the end.