ELI5 Why I should buy bonds?

I am in the process of moving my investments away from mutual funds and into self-managed ETFs, stocks, etc. I admit I knew absolutely nothing about investing until a few months ago.

I’ve now read a few books, and followed the steps on this sub. Per someone’s recommendation, I did the Vanguard assessment of how to invest.

It told me to put X % in stocks and Y % in bonds.

Stocks and ETFs I’m starting to understand and that makes sense to me. Bonds I do not understand at all.

Can someone please ELI5 why to invest some money into bonds? From what I’m seeing at my brokerage the maximum return is like 5%, which I get in Cash.TO.

Help?