More approximately, they can be felt by some particularly skilled fighters. Some warriors are known to be able to adjust their power at will (minimum to their maximum power limit) and due to the complexity and skill of some characters, the moniker becomes less important as the series progresses except in semantic discussions.

The term is less often used in the original Dragonball manga. More often, it's used perjoratively by anime fans to refer to the perceived excessive oneupmanship in some battle oriented anime.

Handy DBZ References

Vegeta is constantly fighting to gain a higher power level than Goku, but never succeeds.

Young Trunks has a higher power level than Goten, but not by much. They can synchronise their power levels in order to fuse.