IRLB3034PBF is the most commonly used mosfet. Is that the one you ordered?
An n-channel mosfet is typically used to switch the low side (negative end of load).
A p-channel mosfet is typically used to the switch the high side (positive side of a load).
RDS ON is the on state resistance of the mosfet. Ideally you would want it to be 2 milliohms (0.002 ohms) or less. Higher than that would result in excessive power dissipation (mosfet heating).
VGS TH is a voltage that when applied to the gate (basically a capacitor, it gets charged) the mosfet will BEGIN to start turning on. The voltage of your batteries need to be higher than the VGS TH value listed in the data sheet.
ID is the max current. The value may be quite high, several hundred amps. But note that is at 25C (about room temp). Due to the resistance of the mosfet (RDS ON) and the resulting power dissipation, without a heat sink the mosfet will be able to handle much less than that (observing thermal limitations). A 3034 without a heat sink is good for approximately 30 amps without overheating.
VGS will be the max voltage you can apply to the gate. Don't exceed what is listed.
I only mentioned a few of the different specs, but those are some of the most important ones for your purpose.
Would recommend you watch this great video on mosfets, goes into some functionality and detail on some of the specs: