From admin console help:
The number of inactive seconds on a reserved connection before WebLogic Server reclaims the connection and releases it back into the connection pool.
You can use the Inactive Connection Timeout feature to reclaim leaked connections - connections that were not explicitly closed by the application. Note that this feature is not intended to be used in place of properly closing connections.
So I believe it's self explanatory, let us know if you need some more light on this.
regards