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. Assembler directives can have many operands, often in the form of a comma delimited list. They are often placed at the end of a line and consist of a brief description of one or a few lines of code. It is possible to have a line that is all comment. In this case the first thing on the line should be the semi-colon, for example.