Sometimes you want to highlight a single word within a sentence. There are two important elements for that: <strong> and <em>.

<strong> for importance

<strong> marks text as particularly important. Browsers display it in bold by default:

<p><strong>Important:</strong> Training is cancelled this Tuesday.</p>

<em> for emphasis

<em> ("emphasis") marks text that you'd emphasize when reading it aloud, similar to stressing a word when speaking. Browsers display it in italics by default:

<p>I <em>really</em> love football more than anything.</p>

It's not just about looks

Just like with headings: <strong> and <em> describe meaning, not just appearance. There are also the elements <b> (bold) and <i> (italic), which look exactly the same visually - but they carry no particular meaning, they just say "make this bold" or "make this italic", without giving a reason.

This distinction matters for people who can't see a website but have it read aloud by a screen reader: a screen reader actually stresses <strong> and <em> text differently in its voice, but often doesn't do that for <b> and <i>, because no particular meaning is marked there.

Nesting both in one sentence

You can use <strong> and <em> multiple times within the same sentence too:

<p><strong>Note:</strong> the next game is <em>especially</em> important,
  because we're playing the league leader.</p>

Now add an important announcement using both elements to Finn's website:

<h2>About Me</h2>
<p>Hi! I'm Finn and I <em>really</em> love football more than anything. I've
  been playing for three years in my club's youth team, and my favourite position
  is <strong>striker</strong>.</p>