The CPU can put a binary number on the address bus, to select an individual register in the ROM or ram or the I/O. The arrows on this bus show that addresses go one way only. Data at the selected address can be put on the data bus.
The CPU can also put data on this bus which can be written into a register of ram or i/o. It is not possible to write data into ROM (read only memory). This is shown by the single arrow on the ROM data bus and double arrows on the other two. The control bus instructs the chips to do various things, such as when to read or write etc.
The clock tells all the chips when to change what they are doing. Like the drill sergeant who shouts "LEFT, RIGHT, LEFT, RIGHT". The crystal control the speed of operation.
In simple systems the crystal frequency is 1,000,000 Hz.