The Most
Most, Extremely
adverb
This is the adverb version of the kanji, so it has the same meaning, basically. You can say this is the most or just most. You can also use it as extremely.
もっとも
The reading for this is a reading you didn't learn with the kanji (the kun'yomi reading). To learn it, let's look at a mnemonic.
Think of the most ridiculous motto (もっと) you've ever heard. Now imagine a motto that's even more ridiculous. Here are some actual totally real mottos that are the most ridiculous: "What Can Brown Do for You?" "It's Not for Women" "If It Doesn't Get All Over the Place, It Doesn't Belong in Your Face" and of course, "Why Not Enjoy the Go?"
げしとは、一年で最も夜がみじかくなる日のことだ。
The summer solstice is the shortest night of the year.
ふじ山は日本で最も高い山です。
Mt. Fuji is the tallest mountain in Japan.
彼、今日の夜何食べたか知ってる？焼き肉食べ放題に行ったんだよ。最も臭いおならに備えておいた方が身のためだよ。用心に越したことはないんだから。
Do you know what he ate tonight? He went to an all you can eat yakiniku restaurant. We’d better prepare for the smelliest farts imaginable. Better safe than sorry.