Oh and by the way, those asking what to do haven't really been answered.
As this is Rush mode only, you have to either Defend or Attack the MCOM stations. There are two at each location (A and B).
When defending, you have to stop the enemy planting bombs on the station. If that happens, you'll hear the alarm. You then have to get to the MCOM that's been hit and deactivate the bomb (holding B or circle). Big points for success.
As attackers, your job is to plant the bombs. Again, head to either A or B and plant the bomb this time, holding down B or circle. There are many tactics, either going for it yourself, providing support or even cover.
The beta is good, but the full game will be epic.