Powerful spirits or gods which have various areas of power (often over nature - wind, thunder, rivers, etc) and / or human life (jobs, love, harvests, succes, etc) and guard the area and people around them. So they may grant blessings, often in the form of omamori sold at a shrine where they are worshiped. The kami have a dual nature where one side is benevolent and protective, and the other is destructive and vengeful.
...any being whatsoever which possesses some eminent quality out of the ordinary, and is awe-inspiring, is called kami.1Kami usually choose to remain invisible to the human eye, but in many of the old stories they choose to appear to humans. It is possible for sapient beings to become a kami after death and for some mythological creatures to assend to kami-hood in their lifetime.
The Swordsmith on Mount Inari by Ogata Gekko,© Victoria and Albert Museum, London
Liminal Chronicles gives a highly simplistic view of the Kami. In real life, shintoism the relation of the Kami is much more complicated and beyond my ability to put it into stories. Here is a super quick overview of Shintoism.
A bit more reading to get you started in your research at 1 Wikipedia.