In addition to @Darren_Murphy’s thorough description…in simplest terms, a helper table is a table that does not store data…at least not long term. It’s a table that does “work” (or performs a specific function).
This is done with the use of user specific columns and/or computed columns. When put together, it allows the helper table to perform it’s intended function and give you whatever result you need.
It’s just a matter of allowing the user to enter data, or using various computed columns to bring data into the helper table from other tables, to allow the helper table to do it’s work.
Sometimes you will see them referred to as work, or working tables. At my company, we have tables that only store data for a short amount of time while a program is running. Once the program is finished, then the table can be cleared, so it’s ready for the next run. We tend to call these work tables.
Another way to think of it is like RAM on a computer. RAM is a temporary place for data to be stored while a program is running on a computer.
I think of them as a creative way to “write code”, or get a “code like” experience in a no-code environment. There are a ton of uses for them.