A toxic backlink is the kind of link you did not earn and do not want. Most of the time Google just ignores them, but a pattern of them can still drag on trust.
The short version
A toxic backlink is a low quality, spammy, or manipulative link that can harm a website's search rankings or trust signals, often coming from link schemes, irrelevant sites, or known spam networks.
Signs a link may be toxic
It comes from spammy or irrelevant sites
It looks bought or part of a link scheme
It uses unnatural, over optimised anchor text
What to do
Do not panic. Google ignores most low quality links automatically. Focus on earning good links, and only consider disavowing if you see a clear, harmful pattern you cannot get removed.
FAQ
Should I disavow toxic links?
Usually only as a last resort, when there is a clear pattern of manipulative links you cannot remove. For most sites, Google simply ignores spam.