From the 64 values in the table on the left, count how many fall within the given ranges under the "classes" column in the table on the right. The "frequency" is the number of values in the data that belong to a given "class".
For example, "< -16.0" means "values below -16.0". Only one number satisfies this: -16.2 (first row, third column). So the frequency for this class is just 1.
Then for the range "-15.9 - 13.0", which probably means "numbers between 15.9 and -13.0, inclusive", the frequency is 0 because every number in the table is larger than the ones in this range.
And so on.