Multi, along with Bias, is one of the two upgrades that make you gain more points per second.

(ladder_position - 1) * multi_count power every second.
Multi can be acquired when the power requirement for it is met. Acquiring multi resets all your current power, points and bias_count but improves the base part of your powergain.
Multi can also be decreased by a successful vinegar throw (see vinegar page for details).
The cost of multi is calculated with the formula (ladder_number + 1)^n where n - number of desired multi. For example, the cost of x4 multi on Ladder 3 will be (3 + 1)^4 = 256 power.
These costs are adjusted on [CHEAP] and [EXPSV] ladders - for more information check their respective pages.