Inverting amplifier beginner question
There may be an amplifier or mixer stage before this. To work properly the output of this needs to be biased at certain dc levels. These levels may not be the same as the base bias for this stage. The capacitor allows the different DC bias levels to be connected but the AC signal to pass through.

A capacitor will block DC but allow AC to pass.  

Can I suggest that instead of starting with some quite complex concepts you work through some of the basic tutorials available on the internet this https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law and the other sparkfun tutorials are written for the absolute beginner.