Explain Like I'm Five

It's rather vague to me too, the most helpful summary I found was this one:

In general, the condition applies when:

• The processing isn’t required by law, but there’s a clear benefit to it;
• There is little risk of the processing infringing on data subjects’ privacy; and
• The data subject should reasonably expect their data to be used in that way.

So "we don't have to do this, and most likely it won't be privacy sensitive, and you probably already know we want to do this, but you can still opt out"

Source: https://www.itgovernance.eu/blog/en/the-gdpr-legitimate-interest-what-is-it-and-when-does-it-apply