What’s under the desk?
Please come down.
I live on the floor below the Tofugu office.
When a vocab is a single kanji with no okurigana (that's hiragana attached to the kanji) it tends to steal the meaning of the kanji it comes from. So, this one is below or down.
When a vocab is a single kanji and is alone it tends to use the kun'yomi reading. You learned the on'yomi reading for this kanji, so now it's time to learn the kun'yomi reading to learn this vocabulary word.
What's below you? What's below your foot? Oh no. The worst thing possible: It's した.