In electrical installation, "first fix" means the parts of the job that are done before plastering and "second fix" means the parts done after plastering. So first fix basically involves laying the cables (in conduits) in floors, walls and ceilings, and putting the "back boxes" of the sockets, switches and fittings in place (the inner part where the cables are connected). When this is done, the walls and ceilings are (re-)plastered and the floors laid or made good, and then the installation is completed in the "second fix", adding the fittings:

"If rewiring work is required, it should be undertaken at first fix stage (before plastering) [...]
As well as installing new cabling, first fix stage will involve fitting new back boxes for all sockets and switches. [...]
Once the first fix stage has been completed, the property can be re-plastered or the walls and ceilings filled and made good, and the flooring replaced. The second fix work can then proceed fitting sockets and switch plates, light fittings, the consumer unit and wiring any electric fans, cookers, extractor hoods, electric showers and the immersion heater, if there is a hot water storage cylinder."
https://www.homebuilding.co.uk/wiring-explained/