Keyword research is used for e.g. in content marketing, SEO and SEA to get more traffic and increase leads and sales. If you want to know, what SEO is, read this: What is SEO? Keyword is a search term, that you look up in Google e.g. best bars Los Angeles. There are many types of keywords. One of them are generic keywords. These are keywords, which have a broad scope and nothing specific to a niche, location etc. For example SEO is a generic keyword. The other type of keywords are longtail keywords like the example above. Generic keywords are mostly very competitive, because many other people are already using it. That’s why for a newbie, it is recommendable to use longtail keywords.
It is a good way to find out, what kind of topic your potential reader want to read, so that you can get targeted visitors to your website. So it is a good way to do market research. There is a tool to find out, which keyword has much search traffic on Google. You also can find out, how much competition already exists for that specific keyword. This tool is provided by Google itself and it is called keyword planner, which is free to use. Very handy!
Here is an example using the keyword planner. As you can see 100k to 1m people search worldwide for this topic SEO. The good thing about it, there is not much competition too, which means it is relatively easy to get your content ranked high with the keyword SEO.
You also can specify the location for example in your area to find out, how popular the keyword is. As you can see there are between 1k to 10k people search for it per month in Los Angeles and there is more competition in this field, which means, there is more content about it. It also can mean, that there are many SEO professionals and SEO agencies in Los Angeles. The higher the competition, the more difficult it will be to be ranked high on the search engine. There are many search engines out there e.g. Google, Bing, Yahoo etc. to name a few.
A similar tool is called Google Trends and it looks like this:
As you can see, currently US Americans are very interested in Trump administration. There are also more tools e.g. Moz, Google Search Console etc.
Update 19th July 2019: A more or less usual way to do keyword research is to utilize Wikipedia.
By typing in a certain topic like in this example marketing, you can see related keywords to marketing, which are also relevant to be in ranked in Google.