To Weber, a social action was an action carried out by an individual to which a person attached a meaning, an action that takes into account the existence and possible reaction of others. In other words, if you do not think about an action, it is not a social action. So an accidental car accident or a sneeze are NOT social actions. Also, if an action does not take into account the existence and possible reaction of others (i,e. no one knows about it but you) it is also not a social action. So (for example) whacking off in the shower or praying in private are NOT social actions.
Social action can be explained, according to Weber, in two levels of understanding (or 'Verstehen'). The first is 'direct observational understanding' (or 'aktuelles Verstehen'). For example, you can see that someone is upset by their facial expression. If you see a person hitting wood with his axe, you know he is chopping wood. However this is just the first level of understanding.
The second level of understanding is 'explanatory understanding' (or 'erklarendes Verstehen'), that is, understanding the social action based on the meaning/motive behind his act. Was the woodcutter cutting to earn a wage, build a fire or to work off anger? To achieve this kind of understanding it is necessary to imagine yourself in their situation to try and get at the motives behind their actions.
To Weber even this level of understanding is not enough: for a full causal explanation it is necessary to understand what gave rise to the motives behind the social actions.
Weber goes on to detail in great length the benefits of bureaucracy and rational authority, but I assume that's off-topic. Source(s):
Haralambos and Holborn sociology textbook