Grey hat SEO sits between white hat (Google-approved) and black hat (clearly against guidelines) techniques. Think of it as bending rules rather than breaking them outright.
Common grey hat tactics include buying aged domains for their authority rather than exact-match purposes, light PBN usage with high-quality relevant sites, aggressive internal linking and keyword optimization that's technically fine but pushy, guest posting primarily for links rather than genuine value, and buying links from real sites without disclosing the commercial relationship.
The risk is that Google's guidelines shift over time, so what's grey today might be black tomorrow. Most grey hat practitioners accept they might need to pivot strategies when algorithm updates hit, rather than building long-term sustainable rankings. It's higher risk/reward compared to pure white hat, but less likely to get you completely penalized than black hat techniques.