Is there a word or phrase that explains that the usage of "we" in some contexts does not necessarily mean "all of us"?

I need to explain to a person that the word "we" doesn't necessarily mean "all of us" in some contexts.

For example, a speaker giving a speech may say,

"What we learned from the Great Depression was that....

Obviously, not every one to whom the speech is being delivered might have learned the same lessons from the Great Depression. Indeed there might be some who haven't even heard of the Great Depression. So the speaker, using the word "we", doesn't really mean "all of us".

How is this usage of "we" described? I know I can simply say "We doesn't necessarily mean 'all of us'", but I want to know if there is a better word or a phrase that can describe this usage of "we".

"We" is a collective term that is usually used to refer to a group of people, including the speaker. Similarly, "they" is a collective term that is usually used to refer to a group of people, excluding the speaker. Both terms can be used very generally or very specifically.

In your example above:

"What we learned from the Great Depression was that...."

The "we" in the sentence actually generally refers to all of mankind/humanity. However, it does not refer to any specific individuals.

The sentence could be rephrased:

"What mankind learned from the Great Depression was that...."

"We" can also be used to refer to "some of us" instead of "all of us".

For example, when talking to his child, a father might say:

"We (your mother and I) are going to the store. We will see you when we get back."

We (the readers of this post and myself) can clearly see that, in the sentence above, "we" refers to only 2 of the 3 people who are present. The third (the child) will not be partaking in the activities performed by those who are part of "we".

This is a nosism; in particular, it sounds like the editorial we, defined by Wikipedia as:

Here, the writer has .. cast himself in the role of spokesman: either for the institution who employs him, or more generally on behalf of the party or body of citizens who agree with the commentary.

Though slightly less accurate, this usage could also be construed as the author's we:

Similar to the editorial we is the practice .. of referring to a generic third person by we [for example], "By adding three and five, we obtain eight.", "We are thus led also to a definition of 'time' in physics.". We in this sense often refers to "the reader and the author", since the author often assumes that the reader knows [agrees with] certain principles.