Fields
Simple explanation
In left + right, both operands are expressions — but they play different roles. Fields name those roles so tools can ask for "the left side" directly instead of counting children.
Technical explanation
language/fields.zig maps production ids to (field name, child index) bindings; fieldIdForName gives stable 1-based ids. Nodes resolve them through childByFieldName / fieldNameForChild, and query patterns constrain children with left: (...) syntax while cursors report currentFieldName(). The bundled grammar binds left/right on all four binary productions.