What does "stagger" mean?

In the Heavy Armor skill tree, the following perk exists:

Tower of Strength: 50% less stagger when wearing only Heavy Armor.

What does "stagger" mean in this context?

Does it do anything if you already have the Conditioning perk?

Conditioning: Heavy Armor weighs nothing and doesn't slow you down when worn.

Solution 1:

You may notice, from time to time, you or enemies will take a few steps back and not act for a few seconds: that's stagger. It's essentially a very short duration stun. Tower of Strength simply reduces the chance of getting staggered by 50%.

Stagger is not affected by Conditioning, which only affects the weight value of equipped heavy armor.

Solution 2:

Yes, it's definitely the chance.

With Tower of Strength + Fus meditation, my heavy armor character rarely staggers.

Note that this does not stop knockdown / knockback - so more powerful enemies like Duegar Deathlords will still send you flying.