Configuring an ASA can be a dauting task if you are not familiar with some concepts that are key to the operation of the device. One such concept is that of security levels.

To summarize this concept, an interface can be assigned a security level of 0-100, such that the security level signifies the level of trust. So typically your inside interface, the most trusted interface, would be assigned a security level of 100 while your outside interface, the least trusted interface, would be assigned a security level of 0. Traffic is allowed to flow from a more trusted interface to a less trusted interface.