Original: IDEA derives much of its security by interleaving operations from different groups - modular addition and multiplication, and bitwise eXclusive OR (XOR) - which are algebraically "incompatible" in some sense.
Simplified: IDEA derives much of its security by interleaving operations from different groups - modular addition and multiplication, and bitwise eXclusive OR (XOR) - which are chosen to be "algebraically incompatible".