what is the use of Smart object

Hi guys , Can anyone have any tutorial for how to use Smart object in photoshop.

smart object = just to maintain the quality of the image pixel size of the original.

you can find tutorials on youtube. : O

Also, If you have a layer as a smart object, you can double click (or right click - edit contents). This will open a new photoshop file and you can swap out the content. When you save the automatically opened file, you can go back to your original file, and this will be what you saved from the other file.
Very much used for mock-ups (google it if you don't know what this is - you will not regret this!)