px stands for pixels and is a way of measuring how big something is on a screen. The higher the number before px, the bigger it is.

<html><body><p style="font-size:26px;">I'm kind of a big deal</p><p style="font-size:13px;">People can read me</p><p style="font-size:4px;">This was a bad choice</p></body>
</html>
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As we'll see when we'll make our own website, we can use px for a lot more than just text.

Make sure to add the number before the word px.