A language object defines how to parse a particular programming language. It is usually dynamically loaded from a shared library (.dylib, .so, .dll).
The variable tree-sitter-load-path is a list of directories that the function tree-sitter-require uses to search for these shared libraries. This is similar to how the built-in function require searches for Emacs libraries on load-path. The default value contains the directory used by the tree-sitter CLI tool.
;; Load the language definition for Rust, if it hasn't been loaded.
;; Return the language object.
(tree-sitter-require 'rust)
The package tree-sitter-langs is a language bundle that contains shared libraries for some languages (as well as syntax highlighting queries). When it is loaded, its shared libraries are prioritized over the CLI’s directory.
Syntax-aware language-agnostic mechanisms are meant to be defined by tree-sitter-mode and its dependent minor modes. They determine the language object to use by consulting the variable tree-sitter-major-mode-language-alist. This list is empty by default, and gets populated by tree-sitter-langs when it is loaded, and by language major modes that are tree-sitter-aware.
Language major modes use these generic mechanisms to provide functionalities specific to their languages. Currently, there are not many language major modes built on top of tree-sitter. An example is csharp-mode.
For the full list of languages bundled by tree-sitter-langs, see the submodules under its repos/ directory.