。
She is currently in a meeting.
Literal
She [topic-は] currently meeting-during is.
会議中 stacks the noun 会議 ('meeting') with the suffix ~中 (read -chuu here), 'in the middle of, during.' The same suffix gives 食事中 ('eating'), 仕事中 ('working'), 通話中 ('on the phone'). Closing with である rather than です or だ marks this as formal written Japanese — exactly the wording you'd see on an internal status board, an out-of-office reply, or a written report. In spoken phone reception, the equivalent would shift to keigo: ただ今会議中でございます or 席を外しております.