「」。
In plain terms, "on-board" means there's no graphics card.
Literal
This on-board [quotative-って-topic-のは] flatly-say-[if-ば] "graphics-card [subject-が] attach-[progressive-ていない]" thing [copula-だ].
Tech explainer style. 平たく言えば ('speaking plainly, in layman's terms,' literally 'speaking flat') is a fixed expression for simplifying jargon. オンボード is the English loanword for 'on-board' (meaning 'integrated into the motherboard'). The このXというのは construction introduces a term for definition: 'as for this X.' The whole is the classic [jargon]+を+[plain explanation] definition pattern.