The post SengKiat suggested to answer Siebe‘s question is good.
I just wanted to clarify it a bit more, because I think this is important.
The sutta is “Dutiya Ssotāpanna Sutta (SN 48.3)“, and is short: “Pañcimāni, bhikkhave, indriyāni. Katamāni pañca? Saddhindriyaṃ, vīriyindriyaṃ, satindriyaṃ, samādhindriyaṃ, paññindriyaṃ. Yato kho, bhikkhave, ariyasāvako imesaṃ pañcannaṃ indriyānaṃ samudayañca atthaṅgamañca assādañca ādīnavañca nissaraṇañca yathābhūtaṃ pajānāti—ayaṃ vuccati, bhikkhave, ariyasāvako sotāpanno avinipātadhammo niyato sambodhiparāyaṇo’ti“.
Translated: “Bhikkhus, there are five indriyas: faith, effort, mindfulness (sati), samadhi, and wisdom (panna). A Noble Person who fully understands the nature of matter (yathābhūtaṃ pajānāti) and thus that those five indriya go through the assada (peak stage), adinava (decay stage), and the nissarana (destruction stage) is definitely a Sotapanna, and will not fall from that attainment”.
There is a deeper understanding here, that involves the five stages of a sankata: udayangama, attangama, assada, adinava, nissarana. The above verse skipped the first two stages, but it is clear that is what is meant.
The link that SengKiat has given is only the introduction of a whole section: “Assāda, Ādīnava, Nissarana“.
The key point is that a given person (just like any sankata) undergoes those five stages, which in this life means one’s physical body. Even though the mental body (gandhabba) could still be in early stages of its human bhava, the physical body ages and dies.
In particular, the brain function starts degrading after the middle age. The five indriya also start degrading as the brain degrades. This is something many people seem to disregard. One’s concentration, energy, and brain capacity will decrease with age, and thus all five indriya will go down in strength and of course will die with the physical body.
There is a sutta that emphasizes the importance of cultivating the Path before getting too old. Once the Buddha was travelling with bhikkhus and showed them an old couple begging in the street. They had been very rich but had lost all their wealth and were now quite old and weak. The Buddha told the monks that they both had the capacity to attain higher magga phala if they made an effort in their young age. Even if they started at the middle age, he told them, they could have attained the Sotapanna stage. But now they both were very weak, and had run out of time.
This is really an important point. We need to make as much progress as possible while our brains are still functioning well.
At a deeper level, the understanding is tied to the true nature of matter (yathā bhūta) that exists in this world, one is said to have the yathābhūta ñāna. See: “Bhūta and Yathābhūta – What Do They Really Mean“.