To complement what is said by inflib:
Kamma beeja are are generated in javana citta. That means it is generated while generating abhisankhara.
It is an energy that powers a future bhava. That bhava can be in the immediate future (pavutti paticca samuppada) or in a future life (uppatti paticca samuppada).
For example, when one (say X) is thinking about a bad action done by an enemy, X starts generating bad vaci sankhara, which gets many pavutti paticca samuppada cycles to run through his mind. The more he does this, the more agitated he gets, because he may start remembering past bad actions done by that person too. If someone is watching X, one will be able to see how gets to an “angry bhava” even in a few minutes. X may not be even saying a single word, but his facial expressions will clearly show him getting mad by the minute. The he could be “born in that angry jati” and may say or do something harsh to that enemy.
– So, that is an example of pavutti paticca samuppada.
Now, if somehow the situation gets out-of-control and he kills the other person, then that particular action is done with a very potent apunnabhisankhara. The kamma beeja created by those apunnabhisankhara could lead to a rebirth in the future in a “corresponding bhava“, like in a niraya (hell).
– That is an example of a uppatti paticca samuppada.
That is a condensed summary. We can discuss in more detail, if there are any questions. For example, “sankhara paccaya vinnana” step gets him to an “angry state of mind”. The that leads to changing of X’s facial expressions with “vinnana paccaya namarupa” and “namarupa paccaya salayatana” steps, where all his six senses are now focusing on generating this “angry bhava“.
It is always to good to analyze various real life situations. That makes these concepts clear.