From the project's README:

Tree-sitter is a parser generator tool and an incremental parsing library. It can build a concrete syntax tree for a source file and efficiently update the syntax tree as the source file is edited.

Qt Creator could use tree-sitter for language-related tasks that need to be fast, like syntax highlighting, code folding, and structural editing.

Code analysis and refactoring could be left to more heavy-weight LSP.

Tree-sitter is an increasingly popular project with integrations in neovim and Emacs and could be maybe used by Qt Creator to support more languages.