I have submitted my paper in one of the Elsevier journals. It was under review, and now it is decision pending for more than 15 days. Does it mean that the paper will be rejected?
"Decision pending" means that the decision... is pending. They haven't decided yet.
There isn't any secret code to deciphering the submission tracker. Things just always take much longer than we think they would.
Since you (nor I) know how your paper's status is changed in the journal's website, you shouldn't jump to conclusions.
It is likely that the change "under review" -> "decision pending" means that referee(s) report(s) were received, and now the editor has to make the decision. In many journals the decision does not fall solely on the editor, but papers are discussed to some extent by the editorial board. Those things often take time.
It could be that the editor is pondering the decision. It could be that the editor only works on editorial decisions every couple weeks. It could be other things.
The way I try to see it, is that things seem to be a lot better than they used to be when there were no online submission and tracking systems. There was a time when we actually submitted the papers by postal mail.