Percent
A percent is a slice of 100. 10% of €200 is €20.
In plain words
"Per cent" means "for every hundred". 25% is 25 out of every 100: a quarter. 50% is half. 100% is all of it.
Up and down are not the same
Go up 50%, then down 50%, and you do not end where you started: €100 becomes €150, then €75. A fall needs a bigger rise to get back.
Percent or percentage points
If a rate goes from 2% to 3%, it rose by 1 percentage point, but by 50% of itself. News often mixes the two up. Ask: points, or percent of what?
Per year
A rate "per year" is a speed. Growing 10% a year for two years is 21% in total, not 20%, because the second year grows on top of the first.