This depends on the market you're trading. 

To work this out, tap on a market you're interested in and tap the "i" in the top right hand corner.

In the market information look for the "Underlying unit risk", this will tell you which part of the price you are trading e.g.

EUR/USD states 0.0001

This means, the 4th decimal effects your profit or loss. So, whenever the market moves from 0.0001 to 0.0002 (for example), you will make or lose that amount multiplied by your "Quantity" or stake size. 

That market movement is also called "points" or sometimes "pips". 

Confused? Let's use an example...

I buy EUR/USD Rolling Daily at £2.00 a point
The price I bought at was 1.18158
Then the market goes up and the sell price is now 1.18168
If I closed my trade now, I would have made +£2.00 profit, because the 4th decimal has gone up by one "point" or "pip"

If the market moves down and the sell price is 1.18148
If I then closed my trade, I would have lost -£2.00, because the 4th decimal has gone down by one "point" or "pip"

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