#Mass.
Mass.
Whether makeshift, or purpose built.
It serves 2 functions, the mass itself is going to reduce the tendency to be affected by the environment, but also it will lower the centre of gravity of the entire structure, enhancing stability.
Whether it is purpose-designed or ad-hoc - sandbags or just your camping pack or camera bag, it will serve the same function.
There are 3 main methods.
Hang from the centre
fastened to all 3 legs, low down - this is probably the best, but requires the correct equipment
or more ad-hoc, draped on one or more legs
This always assumes your tripod can take the total weight, of course.