It doesn't really mean anything anymore. The transistors are not 5nm either. It's just marketing.

Quoth Wikipedia:

The term "5 nm" has no relation to any actual physical feature (such as gate length, metal pitch or gate pitch) of the transistors being 5 nanometers in size. According to the projections contained in the 2021 update of the International Roadmap for Devices and Systems published by IEEE Standards Association Industry Connection, a "5 nm node is expected to have a contacted gate pitch of 51 nanometers and a tightest metal pitch of 30 nanometers". However, in real world commercial practice, "5 nm" is used primarily as a marketing term by individual microchip manufacturers to refer to a new, improved generation of silicon semiconductor chips in terms of increased transistor density (i.e. a higher degree of miniaturization), increased speed and reduced power consumption compared to the previous 7 nm process.