In more detail
Links have been part of how search engines work since the beginning. Google's explanation of how Search works describes how it discovers pages by following links from pages it already knows. Links also act as a signal of how others regard a page, which is why they are valuable and why they are often manipulated.
Not all backlinks are equal. A link from a respected industry publication that mentions your research is very different from a link in a comment on an unrelated blog. Relevance, the linking site's own reputation, where the link appears on the page and whether it was earned all matter. Links marked with nofollow, sponsored or ugc attributes signal that the linking site doesn't vouch for them.
Third-party tools estimate link strength with their own metrics, such as Moz's Domain Authority and Page Authority, or Ahrefs' Domain Rating. These are useful for comparison but are not Google measurements.
Why it matters when hiring an agency
Backlinks are where many SEO problems start. Google's spam policies treat buying or selling links that pass ranking credit, excessive link exchanges and large-scale automated link schemes as link spam, which can lead to lower rankings or a manual action.
Ask any agency to explain exactly how it gets backlinks, show examples and tell you whether money changes hands. Be wary of fixed monthly link counts sold at a fixed price. Before you start, check the strength of your key pages with our Page Authority Checker so you have a baseline to compare against.