That means "same power", in the context of a comparison of some other metric. So (from the article) "15% higher performance at iso-power" means that for the same power draw, a chip made with the new process has 15% better performance (clock speed) than one made with the old process. Likewise for iso-performance: new chip can get the same perf with less power.

Assuming you are referring to this: "N5 provides around 15% higher performance at the iso-power or, alternatively, up to 30% power reduction at iso-performance."

They are just saying that for the same power usage as previous tech, they get 15% more performance, or 30% power reduction while achieving same performance as previously acheivable."

The prefix "iso" means "same"/"equal". E.g. isobars (same pressure) on a weather map, isotopes (same place) on the periodic table, isomorphic (same structure), etc.

Here iso-power means comparing performance for the same power usage, and iso-performance means comparing power usage for the same performance.

Comparison at same power usage. (and iso-performance for comparison at same performance)