Can any one please suggest me how to "archive" the list items (say "related to 2015 year") that are listed in SharePoint. "Archive" means I should indicate that these forms are relate to 2015 year.
It depends what you mean by "archiving". You can move items to another list, or you can just filter them from the view.
Use a list column to hold a value, e.g. a status text or a year number. Then create views that exclude that value.
You can create workflow which can move the items from Main List to archive list.Here are the steps to create a workflow for archiving purpose.
Or You can think about writing a custom timer job which will move the items from main list to archive list.
Or Use the save as template options, Save the Main list as template and create a new list using that template. After that delete the all the items which you dont want in the new list.
If you're worried about having too many archived lists cluttering up your Site Contents, it may also be possible to export the list to Excel, combine the items from previous years into separate sheets (or all into one with the ID's updated), and re-upload the Excel file into a SharePoint archive document library.
I think SharePoint also crawls through the contents of files so that they'll show up in the search bar, but I'm not quite sure if it works at a level higher than the document library/list search bar.