The tool "Curves" is just a box with a diagonal line running from the top right to the bottom left. However, where you raise or lower that line can make all the difference in the world to your pictures. At the least, it can brighten your midtones and slightly darken your shadows which gives your shots more definition or contrast. At the most it can change the look of your picture completely. The top right area is for your highlights, the bottom left the shadows and the middle, well that's for the midtones. You raise or lower the line, by putting your cursor on the line , clicking on that spot, and then performing the action.
In Picnk, there is a curves tool. I'd like you to start working with it manually, first. But if you're anything like me, you'll open the customize tab and try every option just to see what it does.
Vibrance is saturation on steroids.
When you learn to work with Curves and Vibrance, your photographs will start to look 100% better.