This is an assignment statement:
IncrDayVisits=IncrDayVisits*1.06;
The expression to the right of = is resolved, and the result is assigned to the variable IncrDayVisits. The value of IncrDayVisits will be reinitialized to missing for each iteration of the data step, and if IncrDayVisits is missing when the expression is evaluated, the result of the expression will also be missing.
This is a SUM statement:
Year+1;
The variable to the left of + is an accumulator variable. Accumulator variable values are not automatically reinitialized when the DATA step iterates - the values are instead retained across iterations. After the expression to the right of + is evaluated, and the result is summed to the existing value of the accumulator variable (the equivalent of Year=SUM(Year,1), in this case). Summing will ignore missing values, so if Year is missing, the resulting value of Year is 1.