Raids are short, small skirmishes initiated against player outposts. Since every outpost produces goods, there is always something worth stealing at any given outpost. To challenge an outpost, raiders must kill NPC guards in addition to any defending players. Raiders can stock up on an outpost's goods after its defenders have been bested. According to Goblinworks, NPC guards for outposts are "few and far between." Stealing items from outposts disrupts supply chains to local settlements.
If raiders hold an outpost for over 10 minutes, they can then strip mine it. Strip mining increases the output of goods but deals damage to the outpost until it is destroyed. You can read more about raiding, strip mining, and how each affects your hostility rating over on the official post.