A true first is the very first edition of a work anywhere in the world, as opposed to the first edition in one country. If a book was published in London before New York, the London edition is the true first and the New York edition is the first American edition.
How to spot it
Check where and when the book first appeared, then compare the edition you have. Phrases such as "First published in Great Britain" or "First American edition" on a copyright page often signal that another edition came earlier. Translations are never the true first of a book first written in another language.
What it does to value
The true first is normally the most valuable edition. A first edition in a second country, such as a first UK edition of an American book, is collectable but usually worth less, although for some authors the home country's edition is collected strongly in its own right. Where the editions differ in text, both can matter.
See how to identify a first edition and first edition vs first printing, which explain the related terms.