This is suffering is one of the noble truths. But it is not clear what does "this" stands for ? Similarly in the statement "This is the way leading to the cessation of suffering." , it is not clear what does "this" stands for?
We see this phrase in SN 56.11 (Sutta on Rolling Forth the Wheel of Dhamma, translated by Ven. Bodhi).
“‘This is the noble truth of suffering’: thus, bhikkhus, in regard to things unheard before, there arose in me vision, knowledge, wisdom, true knowledge, and light.
‘Idaṃ dukkhaṃ ariyasaccan’ti me, bhikkhave, pubbe ananussutesu dhammesu cakkhuṃ udapādi, ñāṇaṃ udapādi, paññā udapādi, vijjā udapādi, āloko udapādi.
What is "this"?
It's explained in the same sutta:
“Now this, bhikkhus, is the noble truth of suffering:
Idaṃ kho pana, bhikkhave, dukkhaṃ ariyasaccaṃ—
Birth is suffering, aging is suffering, illness is suffering, death is suffering; union with what is displeasing is suffering; separation from what is pleasing is suffering; not to get what one wants is suffering; in brief, the five aggregates subject to clinging are suffering.
jātipi dukkhā, jarāpi dukkhā, byādhipi dukkho, maraṇampi dukkhaṃ, appiyehi sampayogo dukkho, piyehi vippayogo dukkho, yampicchaṃ na labhati tampi dukkhaṃ—saṃkhittena pañcupādānakkhandhā dukkhā.
Also, "All conditioned phenomena are suffering" (sabbe sankhara dukkha) from Dhp 278.
Something you might want to add to your comment, which I upvoted: one of the seals of the Dharma, namely that "All formations are duḥkha" (sarvasaṁskārā duḥkhāḥ). Mar 1, 2021 at 3:08
1@Caoimhghin Added. But it should be "all conditioned phenomena is suffering". In this context, sankhara is not just formations– ruben2020 ♦Mar 1, 2021 at 3:37
Makes sense, and I've heard this said before and don't dispute it. That Gautama Buddha says "saṁskārā" and not "saṁskṛtāḥ" always struck me as a curiosity of the early Prākrit Buddhavacana, if we'll forgive my using Sanskrit instead of Prākrit. Mar 3, 2021 at 18:22
"This" stands for anything that is conditioned or any particular conditioned element.
We know this because that which is suffering is defined in the texts.
In that context, "this" refers to "direct knowledge and seeing". It means you see dukkha, the factors it emerges from, the possibility of prevention, and the way to implement it in real life - you see all of this as it actually is, directly in your immediate first-hand experience.