Full Idea
The best known example of Euclid's 'common notions' is "If equals are subtracted from equals the remainders are equal". These can be called axioms, and are what "the man who is to learn anything whatever must have".
Clarification
'Axioms' are the initial assumptions
Gist of Idea
Euclid's common notions or axioms are what we must have if we are to learn anything at all
Source
report of Euclid (Elements of Geometry [c.290 BCE], 72a17) by David Roochnik - The Tragedy of Reason p.149
Book Reference
Roochnik,David: 'The Tragedy of Reason: the Platonic logos' [Routledge 1990], p.149