The distant sign came into focus as my glasses were adjusted properly.

came into focus 是由动词短语 come into focus 变化而来，come 的过去式是 came。come into 表示"进入某种状态"，focus 意思是"焦点、清晰度"。整个短语的字面意思是"进入焦点状态"，引申为"变得清晰可见"。