The Pāḷi is very straightforward. Yoni means womb. So kammayoni is a “deed-womb”.
The sutta goes on to explain this by saying that beings
shall be the heir of whatever deeds they do, whether good or bad.
AN 10.216
Literally, what womb you are reborn into is a function of your past karma. Figuratively, whatever you become (even in this life) is a result of prior choices.
This opened up a question for me - in English you’d mostly say “your womb” to mean “the reproductive organ in your body” and it’d be a little strange to say it instead of “your mother’s womb” or “the womb”. Is there some feature of pali grammar, vocabulary, or usage, that differs from this?
It’s a subtle distinction, but “My deeds are the womb into which I will be born” is a bit of a different image than “my deeds are the womb in which I gestate my next life”.
In AN 3.77, the simile is similar but conveys kamma as a field instead of a womb. It’s that a person’s deeds, good or bad, create their kamma. One’s kamma determines which realm of existence and the conditions that will take place in rebirth. Consciousness is the seed that is planted in the kamma prepared field. The consciousness creates further kamma with intention and deeds. Craving is the moisture which waters those seeds to grow, mature and ripen, thus continuously repeating the cycle. Consciousness is not a permanent self, but an ever changing process, so the body may break up at death and a new being may be born, but the kamma producing consciousness continues on until ignorance is overcome and kamma either ripens or is cancelled out by wholesome kamma.
‘I am the owner of my kamma, the heir of my kamma; I have kamma as my origin, kamma as my relative, kamma as my resort; I will be the heir of whatever kamma, good or bad, that I do.’
i.e. my origin/birth (from a womb) is due to kamma.
An important part of the message is that you should take responsibility for your actions by body, speech, and mind. Your kamma is not done by someone else or controlled by a higher power. It’s the result of your choices and intentions. Learning to make wise choices and intentions results in blessings. It’s an extremely empowering message. The ball is always in your court.