At the top of a ROBLOX page, when visiting a user's profile, you commonly see a bunch of numbers. If not, you will see a picture, with some text saying "Player cannot be found or no longer exists." (This is known as Error 404, meaning the page is not found. Error 400 also happens, when you search players who are entirely wiped from ROBLOX (such as ID 31042685, known as "Noli" in pop culture of Roblox).
Player ID means that if someone joins ROBLOX, and they are the 100th person to do so, their ID is 100. IDs in the catalog work differently.
This means that if you go to a player's ID, such as InceptionTime, his ID is 7733466 because he is the 7733466th person to sign up to ROBLOX. John Doe and Jane Doe have the IDs 2 and 3 because they were the second and third person to join ROBLOX.
Another example of this is Lethal682, who is an intern at ROBLOX, has an ID of 69468362 because again, he is the 69468362nd person to join.
Some player IDs are swapped between users such as the user "Admin" (ID 18) and "ROBLOX"(ID 1) or the user "Telamon"(ID 13645) with "Shedletsky"(ID 261).
A similar logic as User ID applies to items/groups too. However the logic is a bit different than User ID is because Item IDs, Decal IDs, Audio IDs, Group IDs, and Model IDs are counted differently than user IDs.
For example, if an item's Asset ID is 2463671452, it is the 2463671452nd asset to be created on Roblox.