This is the 13617th most frequent Russian word.
нежелательные
"Unwanted" or "undesirable."
The word 'нежелательные' is used here as an adjective describing 'вещи' (things) that are not desired or suitable.
Эти вещи оказались нежелательными для нашей поездки.
These items turned out to be unwanted for our trip.
In this case, 'нежелательных' modifies 'действий' (actions), implying that these actions are not appropriate or acceptable in the context.
Вы должны избегать нежелательных действий в общественных местах.
You should avoid undesirable actions in public places.
Here, 'нежелательной' describes 'почты' (mail), referring to mail that is unsolicited or not welcomed, such as spam emails.