class SyntaxTree::Kw

Kw represents the use of a keyword. It can be almost anywhere in the syntax tree, so you end up seeing it quite a lot.

if value
end

In the above example, there would be two Kw nodes: one for the if and one for the end. Note that anything that matches the list of keywords in Ruby will use a Kw, so if you use a keyword in a symbol literal for instance:

:if

then the contents of the symbol node will contain a Kw node.