What is secondary Keyword and how to use them?

Hi BHW SEO Experts,
I have just joined in BHW and this is my first post here. As a newbie in the field of SEO. i need help of the expert.

I have came to know a terms "Seceondary Keyword". Actually what does it means and how to implement it with main targeted keywords for better optimization .

For instance "Best DSLR Camera" is the main keyword. what could be secondary keywords related to it.

Should I use the secondary keywords in my single article or make sepearate article…

What is secondary Keyword and how to use them?