What is the /odom topic?

edit

The odom topic represents more then just the "pose" of the mobile robot. If you check the message definition of nav_msgs/Odometry (which is usually used for this kind of message), you see that it also contains the current velocity, as well as the respective uncertainties.

Typically, the odometry describes the "internal" state of the robot, i.e. the integrated position using wheel encoders and, potentially with fused IMU or other sensors measuring internal state. Often it is also used when you fuse a GNSS sensor. Obviously, then you get more in the direction of a localization system then only tracking internal state (see the wiki for this).

About amcl, though... amcl neither subscribes nor publishes the odom topic (as per the wiki). So the answer to

is it ok if i adopt /odom as the robot's localized state

would be no. Seems the odom topic comes from somewhere else. You can use the amcl_pose topic for that, or use the published transform.