Required reviews completed vs ready for decision elsevier
From the outside, it's difficult to tell. Two possibilities that spring to mind are:


The journal requires two reviews, but regularly invites four (this makes sense because reviewers decline to review, never respond to the invitation, agree but don't submit a review by the deadline, etc). Two of the reviewers have completed their reviews, but as long as there's a reviewer who did not explicitly decline still invited, the status remains as "required reviews complete". However if all the reviewers have either submitted a review or have declined to review, then the status updates to "ready for decision".
The journal is set up so that the handling editor makes a recommendation which is then forwarded to the associate editor/editor-in-chief to make a decision. "Required reviews complete" would indicate that the handling editor hasn't made a recommendation, while "ready for decision" means (s)he has.


Either way there's nothing to do except wait.