https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_selectionRange

The TL;DR of the feature is that the server informs the editor about the ranges of syntax constructs at the cursor position (eg, the expression, statement, function, etc at cursor), which the editor than uses to provide smart "extend selection" action which is helfpul to select syntactically self-contained fragment.

This is a relatively important feature to support, as it unlocks powerful editing capabilities.
It also should be relatively easy to add support for, as it only needs information about syntax.