One of the biggest challenges for photographers is learning how to produce properly exposed photos. This is where the histogram comes in, it helps us evaluate the brightness of the image.
The histogram is broken down into black on the left and white on the right with shadows, midtones and highlights in between. The idea is to expose your picture more to the right side.
The example above, from Alwi-Mocind, illustrates how to adjust the histogram. To move to the right something needs to change. It could be changing from f8 to f5.6 for example; or changing shutter speed from 1/250 to 1/125. Each stop you change will mover exposure to right. Conversely, you could dark a picture going the opposite way.
The final thing to mention is illustrated below. 1/2 of the data stored in you picture is one the right side of the histogram in the highlights to white, if your picture is mostly in the center then you only have roughly 1/4 of the possible pixels exposed and it reduces as you move to the left.
I am by no means an expert of this subject, so if you want to really dive into improving your pictures search the internet for more on this topic.