Ergo.Meta.capture

You're seeing just the function capture, go back to Ergo.Meta module for more information.
Link to this function

capture(parser, key, t_fn \\ &Function.identity/1)

capture/2 is used to hoist AST values (possibly transformed by the t_fn) into the context. For example this can be used in a sequence where you want to communicate information forward in the sequence.

Example: You want to parse <a>...</a>. To correctly parse </a> you need to know that <a> was used to open the pair.