In Jainism, "yoni" means the womb or form of birth. The phrase "84 lakh yoni" (8.4 million yonis) is a way to express the vast variety of life-forms a soul can be reborn into due to karma. So, 84 lakh yoni refers to the many possible kinds of beings a soul may inhabit after karmic reactions—ranging from celestial beings and humans to animals, plants, micro-organisms, and beings in other realms.

Notes:

• The exact list and numbers can vary a bit between Digambar and Śvetambara traditions, but the idea remains that there are many possible birth-forms in the cycle of rebirth.
• This teaching emphasizes the importance of ahimsa (non-violence) and ethical conduct, since every life-form is a potential womb a soul may occupy.

If you’d like to read a dedicated Jain explanation on this topic, you can find a page on JainKnowledge.com here: