What is Starlink, the satellite service Iranians use to post about the protests?

Elon's Musk Starlink Satellite service is being used by Iran's protest movement to bypass the regime's internet ban. Although recent reports show that Iranian authorities have been able to block the service locally in some instances. Initially Space X funded the service, but the US Department of Defence later took over the payments. Donald Trump has implied the Washington could do the same in Iran.