The weapons degrade when you die, although this seems to be minor at this point in time. (I have taken quite a few deaths from unexpected blasts and my durability is about what you are seeing as well).
Since quite a bit of your effectiveness is based upon the gear this can be an issue. An example of this is seeing that the quests you are getting are being rated at Very Hard or Devastating. This can be because your gear is either out leveled or simply because your gear is damaged and thus less effective. From the information that I have found however (just found actually), the gear does not scale down its effectiveness. It will function the same when it is 300/300 or 5/300. Its when an item reaches 0 durability that it will stop working and thus why you see quests and such show such a difficult rating.
Fixing the items is fairly easy. Every merchant I have run into at this point in time has a Repair tab on them where you can see a list of your gear that needs repairing. You can then select the items in question or hit the Repair All button. It does cost a nominal fee to repair your gear but I have not seen anything over the top in this area, despite my fairly alarming deathrate.