A table is considered a heap when it is created without a clustered index. Data isn’t in any type of ordered state. Some data is over here, some data is over there.
When you are inserting data into a heap, that data is tossed in wherever. Think of it like your junk drawer. It’s not organized into its own little sections. What do you do when you have something to add such as a pair of scissors or an old pen? You open the drawer, toss it in, and close it up without giving it a second thought.
Like Chad mentions, there are uses for heaps. And when you move to Azure Synapse Analytics, there are more uses for heaps. But with on-premises SQL Server, a heap is usually a mistake.