Both words can be translated as some. But that does NOT mean that they are interchangeable. Here is how to use them correctly.
| Use немного with uncountable nouns. [an uncountable noun, also known as a mass noun, or a collective noun, is a noun that doesn’t occur in the plural such as bravery, love, water, etc.] |
Я возьму с собой немного денег.
I’ll take some money with me.
В стакане было немного молока.
There was some milk in the glass.
| Use несколько with countable nouns. [a countable noun can be counted; it occurs in the plural such as book, coin, tree, etc.] |
У неё в сумке несколько конфет.
She has some candy in her bag.
Я прочитал несколько книг о любви.
I’ve read some books about love.
My Slavic Studies 