The theatre restaurant was closed because internal decoration was in progress.

in 是介词，表示"处于某种状态或过程中"。 progress 是名词，意思是"进展、进程"，来自拉丁语 progressus（前进）。 in progress 是一个固定短语，表示"正在进行中"，强调某事处于进展的状态。