seo – What is @Id in JSON-LD schema?

I am working on JSON-LD schema for SEO purpose and stuck at @id. What type of value can I use? Or is there anything specific value that should I use? Or How it is works? Or How much it is useful? It is totally confusing for me.

<script type="application/ld+json"> {   
"@type": "GiftShop",   
"name": "Example Showcase",   
"@id": ""   
"url": "" } </script>