What does Atticus mean when he tells Scout "to delete the adjectives and have the facts" about Jem's strange behavior in To Kill a Mockingbird?

In order to answer this question, Jem's words deserve to
be read again:

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He declared Egyptians walked that way; I said if
they did I didn’t see how they got anything done, but Jem said they accomplished more
than the Americans ever did, they invented toilet paper and perpetual embalming, and
asked where would we be today if they
hadn’t?

In this context, Jem
is enthralled to be in 6th grade. In particularly he loved the section on the Egyptians.
He, therefore, began to even imitate the way they walked "sticking one arm in front of
him and one in back of him." 

When Scout questioned him
about this, Jem responded by touting the great inventions of the Egyptians. This is
where Atticus' wise words come in. 

Atticus basically tells
Scout to strip down what Jem is saying to it essential meaning. The Egyptians created
some important things that help us even today. In other words, they were
inventive. 

Another way to look at it is by actually taking
out the adjectives, as Atticus advises. If we did that, we would have: paper and
embalming. The Egyptians did invent these two.