Ahhh... Wikipedia :).
OK, so please refer to this one:
https://en.wikipedia.org/wiki/Kashubian_language
Many scholars and linguists debate whether Kashubian should be recognized as a Polish dialect or separate language. From the diachronic view it is a Lechitic West Slavic language but from the synchronic point of view it is a Polish dialect.[12] Kashubian is closely related to Slovincian, while both of them are dialects of Pomeranian. Many linguists, in Poland and elsewhere, consider it a divergent dialect of Polish. Dialectal diversity is so great within Kashubian that a speaker of southern dialects has considerable difficulty in understanding a speaker of northern dialects. The spelling and the grammar of Polish words written in Kashubian, which is most of its vocabulary, is highly unusual, making it difficult to comprehend also written text by native Polish speakers.[13]
Like Polish, Kashubian includes about 5% loanwords from German (such as kùńszt "art"). Unlike Polish, these are mostly from Low German and only occasionally from High German.[14] Other sources of loanwords include the Baltic languages.
As you may see, this subject isn't trivial. If we considered Kashubian as a separate language I would say it is similarly difficult (or easy...) to understand to me as Slovak. But for other ears it could be more difficult.