Previously means at some time before the period that you are talking about.
earlier , formerly
Guyana's railways were previously owned by private companies.
圭亚那的铁路先前为私营公司所拥有。
The contract was awarded to a previously unknown company.
该合同给了一家先前不知名的公司。
a collection of previously unpublished poems.
一个之前从未发表过的诗集。
You can use previously to say how much earlier one event was than another event.
He had first entered the House 12 years previously.
他12年前首次进入了众议院。