Coronaviruses are so named because of the spikes on their surface that make it look like a crown, the Latin word for which is corona. The virus uses these spikes to attach to and enter cells, where they then replicate. All common SARS-CoV-2 variants have mutations in the part of their spike proteins that binds to cells.
The Oxford-Dundee team found that most, but not all, of the common mutations in spike individually strengthened binding to ACE2, a protein found on the surface of our cells.
Furthermore, ACE2 variants found naturally in humans were shown to strengthen binding between it and the virus, suggesting that individuals with common ACE2 variants could be more susceptible to COVID-19 infection.