Short Answer:
This error typically pops up when you’re trying to use a group function, like AVG, SUM, MIN, MAX, COUNT in an inappropriate manner, usually in a WHERE clause. These functions work on a set of rows, not on individual rows, and WHERE clause works on individual rows. That’s why SQL gets confused.

Detailed Answer:
SQL, or Structured Query Language, is a tool used to communicate and manipulate databases. There are certain functions, called ‘group functions’, that are intended to operate on sets of rows to give one result per group. Some examples of group functions are AVG, SUM, MIN, MAX, COUNT, etc.

Now, when you see the error message “invalid use of group function”, it suggests that you are somehow misusing these functions. This usually happens when one of these functions is used in a WHERE clause.

The WHERE clause in SQL operates on individual rows. It checks the condition for each row and if the condition is true, it includes the row in the result set. But group functions, as mentioned before, operate collectively on sets of rows, this misalignment leads SQL to throw the error ‘Invalid use of a group function’.

For example, consider you have a table ‘Orders’ and you run the following SQL command:
“`
SELECT * FROM Orders WHERE COUNT(*) > 5;
“`
This will result in ‘Invalid use of group function’ error. Because you’re trying to use a group function (COUNT(*)) in a WHERE clause. These group functions can be used in HAVING clause once you have grouped your rows using GROUP BY. Or can be used simply in SELECT statement when you’re not filtering your rows based on these functions.

Here’s an example how you can modify the above statement to make it correct:

“`
SELECT COUNT(*) FROM Orders HAVING COUNT(*) > 5;
“`
This is just a simple demonstration. Depending on what exactly you’re trying to achieve, you might need to use GROUP BY clause, or use the function in the SELECT clause. It’s also worth noting that these conditions could be moved to a HAVING clause after your GROUP BY clause.