Relationship between the circuit bandwidth and the gain for an inverter op amp
The two quantities on the graph you refer to are called "unity-gain bandwidth" (12MHz) and "3db point of open-loop response" (24Hz). If you do understand graphs then you can see that for each decade of frequency change the red-line changes by 20dB. This then allows you to predict any points on the line to a reasonable degree of accuracy. 

nb if you know that the op-amp is a simple type of device (20dB/decade roll-off) then you only need one of these two points to predict the other.

This graph (for most simple op-amps) generally applies to any configuration either inverting or non-inverting. When you apply feedback to an amplifier to configure it as an inverting amplifier (for instance), a new line is drawn that indicates the new frequency response of the circuit. This new line, intercepts the slope-part of your graph somewhere between 24Hz and 12MHz and, at frequencies lower than the intercept, the gain remains constant. 

For an inverting configuration |gain| = Rf / Rin where Rf is the feedback resistor and Rin is the resistor in series with your input signal before it connects to the inverting input of the op-amp. If Rf = 10 x Rin then the voltage gain is ten (in dBs this is 20dB).

If you follow the blue line (below) from 20dB on vertical axis to where it hits the red line it intercepts at about 1MHz - this will be the new 3dB point of the inverting circuit. 



So, you went from an op-amp with an open-loop characteristic to a closed-loop characteristic having a stable bandwidth of 1MHz and a gain of 20dB