This is the 7383rd most frequent Polish word.
kinder
In Polish, "kinder" is not a native word but is often used informally to refer to "Kinder chocolate" (a brand of sweets) or sometimes to describe a type of children's play area or daycare (from German "Kindergarten").
Here, 'Kinder' is used as a brand name for a specific type of chocolate, originating from the German-based Ferrero company. It denotes a proper noun.
Dostałem od mamy tabliczkę czekolady Kinder.
I received a Kinder chocolate bar from my mom.
In this context, 'kinder' is used as a comparative adjective derived from the English word 'kind', meaning being more considerate or gentle. However, this usage would usually appear in English structures.
To kinder, aby pomóc innym, jest bardzo ważne.
It is kinder to help others, which is very important.
'Kinder' in this sentence is part of the brand name for a chocolate treat, 'Kinder Joy'. It refers to a proper noun again.