Total keywords is simply a metric that measures the total number of keywords your website ranks for anywhere between 1 to 100 on Google. This metric is pulled from SEMrush.
SEMrush's crawler scours the internet for over 14.9 billion keywords, and finds every keyword that you rank for in billions of locations in 142 different countries, and creates an average for where your website ranks for those keywords. They then report that average as a whole number and add it to your total keywords rankings.
This metric is relative; however, it is a good way to measure your website against your competitors, and track changes and discoverability on the internet over time.