There are situations in your publishing career, where you have a few thousand references that you need to wade through, and cite in your document. For those of us using Latex and Bibtex to create their wonderful research papers, the bib files have a field called the citation key. Now in most cases, the citation key is the Author Name followed by the year. But if your library has grown really large and is synced in multiple places, the citation key sometimes have some funny suffixes. So now instead of a key called “Dabholkar2015”, you would have a key “Dabholkar2015a”. For those using Mendeley and Overleaf as their library manager and Latex editor respectively, the problem is compounded because Mendeley auto-generates the .bib file that you include in Overleaf. Now, since everything is in the “cloud” and “auto-sync”ed, you do not have any way of editing the .bib file in Overleaf. On top of that, Mendeley regenerates the .bib file every time there is a change and so any manual edits you make to the .bib file in your Overleaf project would be lost, the next time, the Mendeley library is refreshed.
Unfortunately, Mendeley have decided that citation keys should not be visible by default, so you have to turn them on. Here I’ll copy from an answer I found on StackExchange : Go to the menu->Tools->Options->Document Details, then choose the document type (for instance, Journal Article), then check the Citation Key box. Then, a field for Citation Key should be visible in the details pane on the right hand side of the window.