ADDI is the assembly language operator for "add immediate" which allows for the immediate increase of a value in a given register by a value instead of what's in another register (unlike ADD which requires three registers for input, this requires only two registers and a value).
Basic format:
ADDI $t1, $t2, 75
Where t1 is the register the new value is to be stored in, t2 is the register containing the original value and 75 is the amount the value in t2 will be increased by.
See also: ADD, SUB, MULT, DIV for other basic assembly arithmatic operators.
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Note: This is all under MIPS architecture. My apologies for not mentioning so before the WRATH OF XP PACK RAPE.