Domain authority is a score from 0 to 100, invented by third-party SEO companies, that estimates how strongly your whole website is likely to rank in search. A brand-new site sits near the bottom; a long-established, widely-linked one sits near the top.
For a small business, it is a useful rough gauge and a common source of confusion. The important thing to know is that Google does not use it. It is a prediction built by outside tools like Moz or Ahrefs from signals such as how many other sites link to yours, not a number Google looks at when it ranks you. You can compare two competitors with it, but it is an estimate of strength, not a ranking you can see inside Google.
Treat it the way you would a credit score: handy for spotting the trend, not worth obsessing over the exact digit. It climbs slowly as real backlinks accumulate and your site earns a longer track record. There is no honest shortcut to a higher number, and no rebuild sets it directly.