Bold Letter, Bold, Bold Character, Bold Text, Boldface
noun, の adjective
What do you think a fat letter is? If I were to guess, I'd say a bold letter.
ふとじ
The reading for this one is a bit odd. The 太 uses the ふと from ふとい (kun'yomi) and the 字 is just the normal on'yomi reading for "letter." You have to remember this weird mismatched reading pair on your own, I'm afraid. Perhaps if you think about how fat letters don't really fit with the rest of the text, kind of like how the readings of both kanji also don't fit in it will help?
この字は太字にしてください。
Make this character bold, please.
これは太字のペンですか？
Is this a bold point pen?
太字ペンでサインして下さい。
Sign with a bold pen.