The add tag accepts two parameters and returns as its output the value obtained after adding together the two parameters.
The parameters may be literal strings, variables or output of other tags.
<cms:add my_count '1' />
The snippet above adds 1 to what ever value is contained within my_count variable and outputs the resulting value.
The tag expects two parameters as the values to be added.
This tag is self-closing and does not set any variables of its own.