Does USB always supply 5.0V
CN2 is probably the barrel connector you are looking for. It is labelled as a "DC_2.0mm" which would be a 2mm diameter DC-jack, which can be known as barrel connectors. CN2 is connected to the circuit GND and the VCC net.

JP12 looks like a jumber to allow you to set the power source which U2 regulator is fed from to generate 3.3V. If pins 1 and 2 are shorted together U2 is fed from the 5.0V net, where as if pins 2 and 3 are shorted together U2 is fed from the VCC net. The jumper will usually either be a set of 2.54mm posts which you can short together using a special shunt piece that fits on top like this:



Or surface-mount pads close together that you can short with a solder blob or a surface-mount resistor like this:



So in this circuit, the USB connector CN1 always provides 5V when connected, but the current available will be limited by the USB host (usually to 500mA). The designer has provided an optional way of supplying your own supply (which could be greater than 5V as long as U2 does not overheat) if the power drawn by the 3.3V circuit is too great for USB. You can choose where to supply your circuit from by shorting the appropriate pins of jumper JP12.