A ResourceLocation is the replacement for numeric id's introduced in 1.8.
It pretty much is a namespaced identifier consisting of a domain and a path which are divided by a ':'. It also supports mods.
To see a block or item id, go ingame and press 'F3 + H', then go in your inventory and hover over an item, it will show the id under the item name.
For a more detailed explanation, see minecraft wiki.
- ID for stone block: 'minecraft:stone'
- ID for a creeper: 'minecraft:creeper'
- ID for a abyss-walker: 'cqrepoured:walker'
- ID for a ice and fire ice dragon: 'iceandfire:icedragon'