I was a reading a book and I came across this sentence "Over time, a sequence of binary digits can be transmitted across a single line. Taken together, several lines of bus can be used to transmit binary digits simultaneously."
If you have a single wire capable of transmitting an electromagnetic signal, then you can transmit 1's and 0's (binary digits) by sending the appropriate signals one at time across the line. If it takes you 10 msec to transmit one binary digit, then over the time of 100 msec, you can transmit 10 binary digits.
If you have a bus (cable), which has multiple wires, then you can send a signal representing a binary digit down each of the wires at the same time, the wires taken together will transmit the digits. If the bus has 10 wires, you can transmit 10 binary digits at once.