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.