the first # would be inches- how long the prop is.

The second # would be pitch. This would be how much forward movement there is. This is the twist in the prop.

If everything being equal- if you increase the prop size in lenght, it will slow the rpm's but increase the thrust.

If you increase the pitch and use a smaller prop, it will lower the thrust but increase speed.

So, your 8x3.8 prop is 8 inches long with a pitch speed of 3.8

When you buy a motor, they usally recommend different props at different voltages. Also using a watt meter will help knowing how far you can push a motor, battery, esc without damage.