This post says
This is all in the context of the data-generating distribution (or underlying distribution of the data). You can check another answer I made for more details on this.
Remove the "in the context of ..." part, the first sentence is "This is all".
Neither cambridge or oxford gives a definition or explanation.
"this is all" is part of "this is all I know" or "this is all I have", what does "this is all" mean here?