There are some good explanations above. Just wanted to add that "это" is unchangeable when it means "there is/there are". The source of all confusion is because unchangeable "это" has the same form as neuter singular (nominative and accusative) of the demonstrative pronoun "этот" (this, this one, that).

это - doesn't change form

этот - declensions (looks like это in Neut.sg.)
Masc. sg. Fem. sg. Neut. sg. Plural
nominative э́тот э́та э́то э́ти
genitive э́того э́той э́того э́тих
dative э́тому э́той э́тому э́тим
accusative э́тот, э́того э́ту э́то э́ти, э́тих
instrumental э́тим э́той, э́тою э́тим э́тими
prepositional об э́том об э́той об э́том об э́тих

In the original Rosetta's example, Это стадион means "This is a stadium" because the woman points to the picture of the stadium.
The verb "to be" is not used in Russian in the present tense, you can sometimes see a dash after это instead of the verb "to be". Это - стадион.

The woman could also give a definition of the stadium like this:
Стадион - это место, где много народу.
Stadium is a place where there is a lot of people.