“Valence” refers to the highest level of orbital(s) where electrons relatively freely wander between structures with similar energy levels. How many electrons this is will vary with atomic size.

1s is the lowest orbital, and has no friends - so helium’s valence shell is just two electrons.

Then there’s a big jump.

2s is similar in energy to 2p, and so electrons will move between the two. To fill this next valence layer you need eight electrons.

Then there’s a big jump

3s is similar to 3p, again giving you an 8-valence level.

Then there’s a big jump.

4s is where you’re now at energy levels high enough to generate a d-orbital, and 4s, 3d, and 4p form all sorts of wacky structures. This layer has 18 in the valence level.

Then… you guessed it… there’s a big jump.

“Full valence” is where these big jumps occur. Adding or subtracting an electron from a full structure is relatively difficult. You’re either forming an entirely new structure on top of the existing one or destabilizing a fully spin-paired configuration. Both are difficult, and so atoms and ions with full valence are highly chemically stable.