Enemies and players that are frozen cannot move or use skills. Freeze has a base duration of 1.2 seconds.
To have a chance to freeze, a skill must have a freeze rate. Most Cold skills have a freeze rate, which is listed on their tooltips.
Your skills' base freeze rate listed in their tooltips can be increased by the "freeze rate per stack of chill" stat, and that final number is then multiplied by the "freeze rate multiplier" stat (which is 100% by default). If the target is frostbitten, it has an increased chance to be frozen. It is more difficult to freeze enemies that have more maximum health and current ward.
Bosses are harder to freeze. In the calculation above, their maximum health is 50% higher. So if a boss has 10 000 health, for the freeze calculation 15 000 is used instead.
A skill with 40 freeze rate and 500% freeze rate multiplier would have a 2.4% chance to freeze an enemy that has 10 000 maximum health and 0 current ward: (40 x 6) / 10 000 = 0.024
If you then apply 3 stacks of chill to that enemy with 50% freeze rate per stack of chill for a total of 150%, your chance to freeze would be 6%: (40 x 2.5 x 6) / 10 000 = 0.06
If the enemy also has 15 stacks of frostbite that add another 300% chance to be frozen, the chance to freeze that enemy would be 24%: ((40 x 2.5 x 6) / 10 000) x 4 = 0.24
If that enemy was a boss, the chance to freeze it would be 16%, as bosses are harder to stun and freeze due to their health being 50% higher in these calculations. ((40 x 2.5 x 6) / (10 000 x 1.50)) x 4 = 0.16