In a toroidal fusion power device, a plasma is confined within a donut-shaped cylinder.
If the gas pressure of the plasma varies across the radius of the cylinder, a self-generated current will spontaneously arise within the plasma, due to collisions between trapped particles and passing particles.
The tokamak uses a combination of external magnets and a current driven in the plasma to create a stable confinement system.
One goal of advanced tokamak designs is to maximize the bootstrap current, and thereby reduce or eliminate the need for an external current driver.
From a kinetic point of view, the bootstrap current is the effect of trapped particles (which practically lie on poloidal plane) and density gradient: the poloidal current due to trapped particles motion is unbalanced since the density is not constant, therefore the bootstrap current is needed to "close the circuit".