I would like to put in a gray box (for example) and inside put a title and below a description. So three boxes in line.

Could you tell me what the code would be like?

Thank you.

If I understand correctly, I think this HTML will do what you want:

<div style="width:100%; margin:0 auto;">
<div style="background-color:grey;color:white;padding:20px;width:200px;margin:0 10px 0 0; float:left;">
  <h2> One 1 </h2>
  <p>London is the capital city of England. It is the most populous city in the United Kingdom, with a metropolitan area of over 13 million inhabitants.</p>
</div> 

<div style="background-color:grey;color:white;padding:20px;width:200px;margin:0 10px 0 0; float:left;">
  <h2> Two 2 </h2>
  <p>London is the capital city of England. It is the most populous city in the United Kingdom, with a metropolitan area of over 13 million inhabitants.</p>
</div>

<div style="background-color:grey;color:white;padding:20px;width:200px;margin:0 0 0 0; float:left;">
  <h2> Three 3 </h2>
  <p>London is the capital city of England. It is the most populous city in the United Kingdom, with a metropolitan area of over 13 million inhabitants.</p>
</div>
</div>

To add HTML : Paste this code to the page where you want to fix this boxes. You can change the width and the background colour of the boxes.

You need to give proper width and alignment to this boxes to fix in a line.

I hope it helps you to solve your query.
Happy working!

Thank you so much @afsanamultanii