What’s the difference? までに vs まで

When by itself, まで means until. But when に follows まで, it indicates a stopping point in time (like a deadline).

5時までに、この本を読み終わらなきゃいけません。 5時まで、この本を読みます。

Go-ji made ni, kono hon o yomiowaranakya ikemasen. Go-ji made, kono hon o yomimasu.

I have to finish reading this book by 5 o’clock. I will read this book until 5 o’clock.