This is all MSSQL info. I do not know how other DBMS work.
A clustered index is the physical order that SQL keeps the rows in a table. Whatever columns are in the clustered index are used to put the rows into a series of 8KB pages where the rows are sorted by the clustered index. If a row is inserted into the middle of the existing clustered index, then rows are shuffled so the new row is in the proper physical order. This is not like a non-clustered index. The columns in a non-clustered index are stored in their own pages in order of the index, with a reference back to the page in the clustered index where the whole record resided. Think of the clustered index as being the table itself.
It is very common that the primary key and the clustered key are the same thing, but it doesn’t have to be.
A heap is a table without a clustered index. The rows are stored in no particular order.
Here is a free class/video on the concepts. If you are working with MSSQL, it is well worth your time, IMO.
Probably not possible to exactly ELI5. 