Semantic Web Systems resources, by Amy Guy, is a remarkably clear-worded introduction to triples, ontologies, and related topics. If, like me, things like RDF have always existed in the “yah, okay, I sort of understand all that enough to get on” space of your brain, this is a good primer. Especially if you’re trying to grok Solid (in the introduction for which I found the pointer to it).
Triples are almost-human-readable statements, made up of [subject, predicate, object]. When you put lots of triples together and there is overlap in subjects and objects, they form a graph; subjects and objects are nodes (circles), and predicates are arcs (lines).