Lamellar (Steel) Question

[Kirbdog]

I am wondering what exactly the speed is.
It says 20ft/15ft. What does this mean for a dwarf?

Lamellar (Steel)
Eastern Medium Armor
Cost 150 gp; Weight 35 lbs.
Armor Bonus +6; Max Dex Bonus +3; Armor Check Penalty -5
Arcane Spell Failure Chance 25%; Speed 20 ft./15 ft.

[Azothath]

Armor, Light, on AoN look at the table. As you go to the right you will see two columns, "Speed (30ft.)", "Speed (20ft.)"
if a creature normally has a speed of 30ft then he gets the new (reduced speed) in the listed column when wearing the armor. Same for 20ft.
  Speed = land speed and sometimes tactical land movement, base speed, or base land speed. It is one area of loose terminology.

at top of page click "Medium"
Medium steel lamellar armor (near bottom of page) reduces 30 ft to 20 and 20 ft to 15.

sometimes if you need the text to go with the rules reading the book or viewing d20pfsrd or a similar website can be better.

[Taja the Barbarian]

Remember that unless they trade it out, Dwarves get the 'Slow But Steady' trait:

Dwarf Racial Traits wrote:

Slow and Steady: Dwarves have a base speed of 20 feet, but their speed is never modified by armor or encumbrance.

So a Dwarf with this trait just ignores the speed restriction listed on medium and heavy armors...

[Azothath]

^ and that's the other half of your question