The most fundamental belief of Islam, and all other principles relate to it. It underpins all other key principles. It is part of the six articles of faith in Sunni Islam and the five roots of Usul ad-Din of Shi'a Islam. It is the central belief contained in the Shahadah, the first of the Five Pillars.
God is close to and involved within the world, closer than the jugular vein and vital to human existence. Helps us, inspires us, gives our lives purpose. Life without God worse than death. Within the world, helps/guides.
God is above and beyond human understand, so it can be difficult to fully understand God. Outside time and space. Eternal, limitless and omnipotent. Not affected by world - He is greater than the world itself.