Bridge has multiple personalities: http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1
So bridge interface is a particular bridge port (the the router-facing port of the switch), assigned with L3 properties (such as IP address, etc.). If VLAN filtering is in use, then bridge port will carry one (or more) VLAN interfaces and in that case, bridge won’t act as interface (towards ROS).