Frozen units do not move at the start of their owner's turn. Instead, they will unfreeze and remain in place. Any units behind a frozen unit will also not move as they are blocked. Frozen units can still be pushed, though this results in visual bugs. Freezing a unit also makes it immune to being forced to attack.
Additionally, as long as they are frozen, frozen units' abilities are disabled. They will not activate when they would usually be triggered. This includes on-death abilities, attacking abilities, and surviving-damage abilities.
Visually, frozen units have icy mounds and spikes around them.
Freeze can be an effective stalling tool and can also lead to devastating combos. However, it has its limitations.
Freeze is a great tool for manipulating enemy positioning. You can also use it to trigger your on-attack cards' abilities multiple times.
When used together, freeze cards can allow for huge damage and destruction, but they can cost a lot of mana.
High single-target damage and high area damage both are things freeze excels at.
You can also use freeze to nullify the abilities of enemy units.