This is the 5098th most frequent Russian word.
мыслей
The word "мыслей" is the genitive plural form of "мысль," meaning "thoughts."
Here, 'мыслей' is the genitive plural form of 'мысль' (thought), used to denote the amount ('много' - many) of something.
У меня есть много мыслей о будущем.
I have many thoughts about the future.
In this case, 'мыслей' is also in the genitive plural but used in context with 'новыми' (new) to describe possession or addition.
Все эти книги наполнили мою голову новыми мыслями.
All these books filled my head with new thoughts.
Here, 'мыслей' appears in the genitive plural, indicating the absence of thoughts when combined with 'без' (without).