Why it matters
As a benchmark against competitors, it tells you roughly how hard a fight each keyword will be and whether your site's credibility is growing. Treated as a goal in itself, it leads to vanity link-chasing: the score is a thermometer, not the temperature.
How it works
The tools crawl the web's link graph and score each domain by the strength and quantity of links pointing at it, logarithmically: moving from 20 to 30 is far easier than 50 to 60. Different tools, different scales; only compare scores from the same tool.
What to do about it
Check your score against the competitors who outrank you: it explains a lot about what's winnable now versus later. Then go back to earning links for reasons a human would care about.
