First fix usually refers to all the work that takes place before you get to plaster your internal walls and ceilings; the exact definition needs to be agreed to with all on site.
Once the structure is erected and you have walls, a roof and upper floors in place, your plumber, electrician, and gas installer if you have any gas appliances (e.g. hob, stove), will start running their services.
There will be a lot of coming and going from these trades; it is their busiest time on your site. They may rely on each other for different aspects and if going direct labour you will need to be kept in the loop.