...I genuinely wondered what tree was involved. Took a Google search before I understood that it was industry jargon, which meant this: to reduce/minify code.
There are many similarly opaque things in the world of web-development.
Every industry has things that have names that you might need to learn.
Much of this was because real compression methods like zip simply weren't available during early decades of web development.
Obviously, if shrinking the code size is desirable, and you were building a library that did minification, you'd want to move on to also adding other, more dangerous changes that could change the meaning of the code, like removing unused CSS classes, doing DCE on javascript, etc, etc.
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Since programming is a giant, decentralized soup of autodidacts, there's no "word of god" authority that can really say "this is the one true name of something". It's mostly just lingo that passes in and out of various communities, and a lot of times communities (as you're seeing in splintercell's comment) try to helpfully match themselves up and standardize so it doesn't degenerate into pure chaos.
Basically remove everything not connected to the root.