A bio-shield was a device that could allow an individual to present themselves as human to scanners, such as the Thirteenth Doctor's sonic screwdriver and the Judoon's scanners.
The Doctor speculated that Lee Clayton was wearing a bio-shield to prevent his undercover actions to protect Ruth Clayton (the human form of the Fugitive Doctor) from being discovered by Gat and the Judoon. The bio-shield worked and the Judoon were unable to differentiate Lee from humans when scanned. (TV: Fugitive of the Judoon [+]Vinay Patel and Chris Chibnall, Doctor Who series 12 (BBC One, 2020).)