It is a tool used in the keyword research practice by SEO professionals to find and research alternative search terms that people enter into search engines while looking for a similar subject. Search engine optimization professionals research additional keywords, which they use to achieve better rankings in search engines.
A Keyword research tool finds and analyzes search data/search terms that people enter into search engines and gives you the insight to data you can use to better inform your strategy. You can use this to understand what people search for, their search volume and the format of the content they are searching for.
Yes most keyword research tools are free for use with the most common limitations being things like limited ranking data and insights. Some have limited publicly accessible data so you will have to sign up for a paid version to have more access to the data.
Keyword research tools help you save time and money by giving you relevant keywords and content ideas that you can use and implement in your strategy, they also provide a sense of direction in your strategy efforts. You can also further your research by entering a competitor’s domain to get a better analysis on them.