We use mnemonics in assembly language because they are easier to remember than numeric opcodes. A directive mnemonic is a word or abbreviation for an assembler directive. An assembler directive is an direction for the assembler to do something as it assembles the program. The first example above is an assembler directive, .word. The second example is the add word instruction.
Operands are what the directive or instruction act on.
In the first example the operand is the constant 0. This operand provides a value for the assembler to place in the storage created and named SumOfDivs. Since addition is a binary operation the addw instruction requires two operands. Instructions generally have two, one or zero operands. There are a few special instructions that have three operands.