It's a standard term. I'll assume you're familiar with 'cousin' (in the strict English/western sense), aka (rarely, only really in contradistinction) 'first cousin'.
1. Your parents' [first] cousins' children are your 'second cousins';
2. Your parents' second cousins' children are your 'third cousins' (and so on);
3. Your parents' [first] cousins are your '[first] cousins once removed';
4. Your parents' second cousins are your 'second cousins once removed' (and so on);
5. Your parents' [first] cousins once removed are your '[first] cousins twice removed' (and so on).
