Yeah, what Broderick said.
Stats increase by 50% every stage.
So + 2 doubles your stat.
+ 4 Triples your stat.
+ 6 quadruples your stat.
+6 is the highest you can go.
So we'll start with a pokemon that has a 350 speed stat.
A +1 Speed boost will be the stat 350 x 1.5 = 525 speed stat.
A +2 boost will result in 350 x 2 = 700 Speed stat.
So it depends on what the actual stat you boosted or reduced is.
Then you just times or divide the stat by however many stages it's been boosted or reduced.
It works the exact same way with moves that reduce stats as well.
Also you can tell the difference between moves that reduce or boost 1 or 2 stages.
A +1 boost or reduce will say Speed rose or Speed dropped.
A +2 boost or reduce will say Speed sharply rose or speed sharply dropped.
Something along those lines.
Here's the full list.