A positron was a particle. It could be found in anti-matter. (TV: Planet of Evil) Positrons could be produced by particle accelerators and stored in electric and magnetic fields. (TV: The Tsuranga Conundrum)
When a positron came into contact with certain fuel materials, as in an anti-matter drive, heat would be produced, which could then be used to generate thrust. (TV: The Tsuranga Conundrum)
Photons could be produced by the recombination of an electron and a positron. (TV: Time and the Rani) An electron and a positron could also be combined to create orthopositronium. (AUDIO: The Time of the Daleks)
A fixed-atom force field created a shield of positrons which would painfully repel any intruders. (AUDIO: 1001 Nights)
In 2023, the Fourteenth Doctor shatterfried the positrons, contained in the Beep the Meep's neutron drive star cruiser, in order to reverse the ignition of the Dagger Drive. (TV: The Star Beast [+]Russell T Davies, adapted from Doctor Who and the Star Beast (Pat Mills and John Wagner), Doctor Who 2023 specials (BBC One, 2023).)