There are many tutorials on the I2C bus that you could access:
...however the diagrams you posted show how the clock signal (SCL) corresponds to the individual data bits in the data signal (SDA) where the status of SDA is read at the 'top' (flat part) of the clock (SCL) signal.
The data could be EITHER zero or one - that's why the image shows 'both' states - it doesn't assume any particular condition when explaining how the data and clock signals relate to each other.