I thought I’d say this again since it keeps coming up.
There are certain techniques which once upon a time could shoot up your rankings very rapidly but which involve tricking Google. They violate Google’s rules and will crash your rankings and possibly get your site completely blacklisted by Google.
These are called “black hat SEO” as opposed to “white hat SEO” which works with what Google is trying to do and follows their rules.
One of these tricks is to put a lot of keywords at the bottom of the page, possibly repeated multiple times, in the same color as the background so that they are invisible to the person visiting the website.
That was a very popular technique 4 or 5 years ago. Now it’s a quick route to ranking disaster.
Don’t do it.