How to read digital waveform?
The individual bits can be either high or low, depending on what device you are addressing, whether you want to read or write and the actual data that is read/written.

EDIT:
Let's first consider the transition between the start condition and the A6 bit. You can clearly see, that on start condition SDA must be low, but then, depending on the actual address, it can go either low or high.

Between A6 and A5 you have two lines crossing: one going low to high, another one -- high to low. Strange enough, it doesn't necessarily mean that the signal has to change. In fact, it can either stay low, stay high, change from low to high or the other way round.

After R/W, again, SDA should go low (or stay low, if R/W was 0), regardless of the data you are transmitting and the address. In case it was high, it would indicate that there's no slave with given address or that an error occured.

2nd EDIT: Think of these address and data bits as qubits: in the world of datasheets, they can be either low or high, it doesn't really matter. But if you were observing a waveform captured by a logic analyzer, you would see that they have defined state.