This is the 12690th most frequent Polish word.
nielegalnych
"Nielegalnych" means "illegal" or "unlawful" (plural form) in English.
The word 'nielegalnych' describes practices (praktyk) that are against the law or regulations.
Nielegalnych praktyk w biznesie należy unikać.
Illegal practices in business should be avoided.
Here, 'nielegalnych' qualifies substances (substancji) as being prohibited or unlawful.
Policja przechwyciła transport nielegalnych substancji.
The police intercepted a shipment of illegal substances.
In this context, 'nielegalnych' is used to describe the immigrants (imigrantów) who lack the proper legal authorization.