Clock speed or rate is the speed at which a microprocessor executes instructions.
In other words its like how much work you can do per second.
For clock speed this is usually measured in GHz or MHz.
One GHz represents 1 billion cycles per second.
One MHz represents one million cycles per second.
Lets say you buy a processor, the manufacturer sets that processor at a certain clock speed depending on the processors model or architecture and the consumers needs. These can range substantially but are usually found currently between 1.5GHz-3.2GHz in retail. So lets say the one you bought is running at 2.5 GHz, thats equal to 2,500,000,000 cycles per second!
Overclocking is when you increase this clock speed from the manufactures "stock" speed. Your forcing the processor to run faster than the manufacturer has made it run. This has can have its benefits, your making it run faster than from what you bought ;D! The negative with overclocking is that since you have tampered with it your warranty may be void and now that the processor has more stress on it, it can run hotter, have a shorter life span or even stop working.