Within this line:
/interface bridge vlan add bridge=testbr tagged=testbr,ether24 vlan-ids=1003
You see testbr appearing twice, and the two instances represent two different things. The first occurrence is the bridge that contains the physical ports and acts like a switch with a number of ports that have been added to it under /interface bridge ports. For the second occurrence, “testbr” acts as a port. It’s the port on that “switch” that connects to the main CPU. It’s treated as a port, like ether24, and is added to the tagged list of VLAN 1003, which means it’s a trunk port for VLAN 1003, transporting tagged frames between the “switch” and the main CPU. Without “testbr” in “tagged” the CPU has no access to VLAN 1003, which means layer 3 constructs like IP address are not available to the CPU on this VLAN.
You can read this post for better explanations http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1