ctrl-y is the Readline 'yank' command which retrieves the kill buffer.
ctrl-u kills from the point to the beginning of the line (similar to OSX/bash
ctrl-_ which clears the whole line). The Emacs-like Readline bindings are enabled in bash and a subset are enabled in many other places (browser search bars etc).
ctrl-u kill from start of line to insertion point
ctrl-k kill from insertion point to end of line
ctrl-w kill previous word (and prepend to kill buffer)
ctrl-y yank contents of kill buffer