High numbers — or high series — are the cards released later in a set's production run, typically carrying higher card numbers. Because they came out later and often had shorter print runs, high number cards can be harder to find than their low-series counterparts.
For set collectors, high numbers are often the final frontier — the cards that make completing a set genuinely challenging. That difficulty adds to the sense of accomplishment when you finally track down the last few high numbers to close out a build.
In vintage sets especially, high number cards command premiums precisely because fewer were printed and distributed. If you're hunting vintage, always check whether the card you're after is from the high series — it might explain why it's been so hard to find.