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, ...
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.