What is Bhava (becoming)?

In paṭiccasamuppāda there are two similar terms: bhava and jāti. Literally bhava means birth where jāti has the same meaning. However these are two different terms. The Buddha explains the term bhava in AN3.76 about the term bhava(see also AN3.77).

Bhava means the formation of kamma to experience the result(vipāka).The formation of kamma to result a rebirth in a kāmaloka is called kāmabhava. Similarly rūpa and arūpa bhava. When there is upādāna there is the formation of kamma to experience the kammavipāka. Therefore the birth occurs accordingly. When there is bhava to be born in kāmaloka they will be reborn in a kāmaloka.

bhavapaccayā jāti
Continued existence is a condition for rebirth

When there is continued existence there is a birth. If there is no continued existence there is no (re)birth.

If bhava is the seedling;

So, Ānanda, deeds are the field, consciousness is the seed, and craving is the moisture.
Iti kho, ānanda, kammaṃ khettaṃ, viññāṇaṃ bījaṃ, taṇhā sneho.
The consciousness of sentient beings—hindered by ignorance and fettered by craving—is established in a lower realm. That’s how there is rebirth into a new state of existence in the future.
Avijjānīvaraṇānaṃ sattānaṃ taṇhāsaṃyojanānaṃ hīnāya dhātuyā viññāṇaṃ patiṭṭhitaṃ evaṃ āyatiṃ punabbhavābhinibbatti hoti. (AN 3.76)