Literally the entrance or doorway of a building; can also refer to the threshold itself. Often used in literary or slightly formal contexts.
彼はとば口に立って、中をのぞき込んだ。
He stood at the entrance and peered inside.
Figurative use: the very beginning or outset of something, like standing at the threshold of a new endeavor.
これはまだ成功のとば口に過ぎない。
This is still only the first step toward success.
Standard kanji-kana mixed spelling.
Kana spelling is also used, especially when the kanji 口 is not needed.
Compound of とば (meaning 'door' or 'entrance', from 戸端) and 口 (mouth/opening). The exact derivation of とば is uncertain, but it is related to 戸 (door).