A global module is used in multiple templates such that when edited once, it shows up on every website page it is used. A global module in HubSpot websites and HubSpot CMS themes could be a website footer, unlike the modules commonly used in templates, where a change affects only that template.
To clarify, if you change a global module, it will affect all templates in which it is used.
Just make sure when you edit these global modules, that you have everything set right.
That is to say, any editing mistake can show up on every page too. Its visibility and universality is the major difference between any normal module and a global module.
Other normal HubSpot modules only show changes where they are used. For example, you can edit the gallery module differently for different pages. But it is not the same with a website footer or header.