What is a Smart Object in Adobe Photoshop?

that is a part of the layer of adobe photoshop you can edit it and remain untouch in Adobe Photoshop CC but it depends for example you can't edit a smart object using content aware scale...

If you need to edit the smart object you should rasterize the layer in order to do so.

what if you resize the image or group of layers in photoshop? you can't get in actual resolution or resize again as pixel will be teard off so smart object is particularly helpful in this scenario.

Just conver your image or group of layer into smart object & resize it freely without caring of pixel broke down you can edit the smart object to edit the actual image or group of layers & save it back.