One
numeral
As is the case with most vocab words that consist of a single kanji, this vocab word has the same meaning as the kanji it parallels, which is one.
When a vocab word is all alone and has no okurigana (hiragana attached to kanji) connected to it, it usually uses the kun'yomi reading. Numbers are an exception, however. When a number is all alone, with no kanji or okurigana, it is going to be the on'yomi reading, which you learned with the kanji. Just remember this exception for alone numbers and you'll be able to read future number-related vocab to come.
一グラム
one gram
一ページ
one page
一ドル
one dollar
このレモン、一キログラム！？大きいですね！
This lemon weighs a kilogram!? That's big!
レベル一です。
It's level 1.
このメロンはアメリカ一の大きさです。
This is the largest size melon in the United States.