Russian prefix не- comes from нет and works just as English prefixes un-, non-, in- etc. When you see a word starting with не-, there is a very big chance it means something negative. If you know what this word would mean without не-, you can guess the meaning of it with the prefix.
Some examples:
Adjectives:
небольшой - small
невысокий - short, low
неженатый - single, unmarried
неважный - unimportant
непонятный - incomprehensible
некрасивый - ugly
некоторый - some, unspecified
невероятный - incredible, impossible
нежирный - low-fat
Pronouns:
некто - somebody
нечто - something
Adverbs: