Hi.
It's a bit difficult trying to understand circuits as a beginner. You should download a copy of the 555 timer datasheet, the astable circuit you posted will be covered very thoroughly in the datasheet. If the datasheet is a little hard to understand you might want to do some reading on basic electronics, ie what resistors do, what capacitors do etc.
By assembling the circuit and making it work you've already come a long way towards understanding it, that's a good way to learn too. I'm sure lots of people blew up some 555 timers when they first started.
And speaking of that you might want to put a 1k resistor in series with your 1meg pot. So you don't blow up your 555... (the datasheet will explain).