Basically just that- avoiding washing your hands after using the toilet means allowing your hands to be contaminated with faecal bacteria and sometimes viral particles (e.g. Norovirus). Even if you're only peeing you're touching the flush, the door handle, the door lock etc. which invariably have other people's faecal bacteria on them and sometimes viruses like Norovirus or Rotavirus. You will then be touching things that other people have to handle such as computers, trolleys, cash, check-out desks etc. so other people have no choice but to get those germs on their hands too. Some of those people will be vulnerable, immunosuppressed or young children who could become severely unwell with a simple infection.
By avoiding washing your hands before preparing food you are spreading whatever is on them (so faecal bacteria from the toilet, or other people's faecal bacteria from the shopping trolley or the self check-out or the door handles at work or train buttons etc.) onto your food and onto the food of the people you are cooking for!
Please be considerate and wash your hands for 20 seconds in these situations. If everyone did it then it would massively cut down rates of norovirus and rotavirus and of bacterial food poisoning.