I heard previously done work can be put in background in your new work but how about if work is not published yet or under a journal review? Should we refer or not and if yes then how?
Yes. In both scenarios, you refer to the material as being "Unpublished". This is not ideal as the reader cannot find it, so update this in your paper at proof time if you can.
I believe it is safer not to include any work that is currently 'under review' but if it has been accepted - 'in press' would be ideal to include if it adds value - however, as Ian said keep an eye, as until the paper becomes final to be published, you might have received the volume and issue numbers with page numbers.
Always use your previous work when they are relevant. Like it has been said above, use 'in press' for when you have been accepted but not yet printed, and put a note 'under review' for when you're still awaiting response.
For those papers that are unpublished, make sure you have them somewhere accessible to readers, like at ResearchGate, and make that known to the reviewers.
If people really want to find your previous work, make it accessible online.