Short version : Both registers are added together and the result is written to the destination register (SRC1).
Long version : The source register (SRC2) and the sign-extended immediate field (Imm16) are added and the result is written to the destination register (SRC1).
The result is truncated to the 32 lower bits. No carry bit is generated or directly available. However, when no carry condition is detected, other versions of this instruction can skip 1, 2 or 3 half-words ADDS1, ADDS2 and ADDS3).