。
Let's get to the main topic.
Literal
Main-topic [target-に] enter-[volitional-よう].
A typical meeting-opener or topic-shifter. 本題 ('main topic, the point') + 入る ('enter,' meaning 'to get into') + the volitional ending ~う ('let's'). The volitional form of a godan verb shifts -u to -ō (入る → 入ろう). に marks the destination of the action.