In other words, they're composed of many different frequencies. The spectrogram breaks sounds into these frequencies. Loosely speaking, each dark band represents a frequency. Each band is called a formant. These are numbered from bottom to top.
Formant transitions are places where there's a sharp rise or fall in a formant.
Generally, these correspond to consonants. A steady state is a place where there is little change in a formant. These generally correspond to vowels. As you go from left to right you can see how the sounds change in time. Now that we know how to read these, we can look at some topics in speech perception.