1+1

2

If you meant something else:

• In binary (base 2): 1 + 1 = 10 (which is 2 in decimal)
• In Boolean logic: 1 OR 1 = 1; 1 XOR 1 = 0
• Mod 2 arithmetic: 1 + 1 ≡ 0 (mod 2)

Tell me the context if you want a specific explanation.