The given answer is good, but I want to clarify that the county (like Hallands län) is not necessarily the relevant higher division, even though you might get that impression from designations like "Tvååker (N)" where the name of the parish alsways goes together with a county code.
The parish is of utmost importance in Swedish genealogy. The parish named "Tvååker" is part of many different bigger units. It was part of "Himle härad" (= hundred). Clerically it was part of "Varbergs kontrakt" (~ deanery) which was part of the Diocese of Gothenburg. It is part of the province "Halland". It is also part of the county named after that province: "Hallands län", which includes most of Halland and parts of Västergötland and Småland. A härad is part of a landskap, not a län.
If you are doing standard lookups in the church archive of Tvååker none of those larger units matter, except for the location of the archive. It is Tvååker parish being in "Hallands län" that has caused that archive to be located in Lund. Most of the time you will only access the parts of the archive that are digitized anyway, and then it doesn't matter where the books really are.
When bigger divisions than the parish are relevant it can be different depending on what clerical, administrative, military, etc. aspect you are using.
What I'm trying to get at is that the county is not so all-important as some are led to believe. It is not at all like US states, for example. The counties are certainly often relevant, but there are also other divisions that ignore county borders.
So why do you see things like "Tvååker (N)" so often? It is mostly because parish names are not unique. There are often several parishes in Swedish with the same name and to differentiate which parish you mean often the härad, the landskap or the län is added. (That is actually not needed for Tvååker, since it is the only parish with that name.)
The län/county is practical to use for that since they have short codes, so it is very common to write "Tvååker (N)", but it is certainly not the only way. When Swedish Wikipedia differentiates between three parishes with the same name "Näs" for example, it is done by province.
https://sv.wikipedia.org/wiki/N%C3%A4s_socken
The Swedish national archive *sometimes* uses counties to differentiate between two church archives for two parishes with the same name.