It can be anything.
-- In a planet's orbit, it's gravitation, between the planet
and the sun.
-- In an atom, it's electrostatic, between the positive nucleus
and the negative electron.
-- In a yo-yo, it's the tension in the string.
-- Inside a car that's turning a corner, it's the friction between
the seat of your pants and the upholstery.
-- On a merry-go-round, it's your grip on the horse you're sitting on.
