What does overlaping indicate in i2c timing diagram
Where the SDA line is shown with a line at the high and low level at the same time, it means the data could be either high or low (but must remain at that level) during that time period. Whether it's high or low depends on what data you are transferring, and will change from message to message. 

Where the lines are shown dashed, it means there are bits of the message that aren't shown because they are similar to the ones that are shown. In this diagram, bits 3, 4, 5, and 6 of the message are skipped over with dashed lines to allow two bytes of message to be shown clearly in the diagram.