Liked the questions - same ones as I've asked myself:
How can I understand UML in plain-English way, enough to be able to
explain it to my colleagues? What are the canonical resources for
understanding UML at a ground level?
Here is what I have found:
For a kick-start: my choice would be Fowler’s *UML Distilled*.
It really is a distillation of the basics, as has been mentioned: definitions, examples, advice on when a certain type of diagram should or should not be used. It is also a good reference, if you want to focus on a certain part of UML without reading the book cover-to-cover.
For a more detailed, yet plain-English introduction: UML 2 for Dummies has done for my colleagues and me.
It not only introduces UML, its syntax and uses at length, but has a lot of advice on good programming and design practices.
There are occasional differences between the two books on what syntax belongs to which version of the UML standard. These however are minute and definitely not essential for using UML diagrams to communicate design ideas.
(For example: whether UML 2 allows discrete multiplicities, i.e. showing that a certain property may have exactly X, Y or Z objects, rather than just zero, one, many or more than X, say; when participants’ names should be underlined...)
For a totally non-academic and less wordy introduction: this blog has articles on various bits of UML:
It's not a textbook, so is far from exhaustive, but also uses non-textbook stories and examples, which are relatable to. The few available posts are focused on introducing UML concepts visually, so you can skip the reading of the text altogether.