On MacOS, what character does `Esc+` or `+Esc` correspond to in Bash, when using iTerm2 terminal?

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[+5] [0] Shuzheng

[2020-04-16 09:28:54]

[ bash shell gnu readline macos ]

[ https://unix.stackexchange.com/questions/580428/on-macos-what-character-does-esc-or-esc-correspond-to-in-bash-when-using ]

I've read that mapping Left Option key to Esc+ should fix many of the issues users of Bash, on MacOS, have with GNU Readline key-bindings.

However, I've never encountered Esc+ or +Esc before, so does it mean Esc, Meta, Ctrl, or something else? How can I test what key my Left Option is mapped to after enabled Esc+ or +Esc?