We call it "niche marketing" it means the specific area you are marketing to.
One of my areas is gardening -- I have been gardening for 20 some years on my own - and professionally for 12 years total. So I am an "expert" in gardening -- I have used and can review many tools, seeds and other products that are useful to gardeners.
If you blog about your young children --- maybe you are an expert on diapers or other baby items. Whatever you blog about or are interested in you can become an expert in and right articles that are rich in content for your blog and then use those to market items from different companies like Amazon.
By using niche marketing you target an audience and since you are an expert in that area they will likely listen to your thoughts and hopefully click your links to check out your products.
