Anybody who knows/can explain what this character means?
2 Answers
$\begingroup$The symbol is symmetric set difference,
$A \oplus B = (A-B) \cup (B-A)$.
The red colour portion shows the symmetric difference.
This is symmetric difference:
$$A \oplus B:=(A-B) \cup (B-A).$$
It is the collection of elements in either of the sets, but not the intersection. Associativity is probably easiest using:
$$(A-B) \cup (B-A)=(A \cap B^c)\cup (A^c \cap B),$$
Where $A^c$ denotes the set theoretic complement.
$\endgroup$
