The sentence has a parenthetical clause in it. That clause can be put between commas, between dashes or between parentheses (braces in BE). It can also be removed without changing the grammar of the sentence.
The clause is "with winds above 155 mph". It adds information about the noun before it ("the strongest").
Here is the sentence, first with the clause in parentheses, then with it removed:
Hurricanes are classified according to wind velocity, the strongest (with winds above 155 mph) being Category 5.
Hurricanes are classified according to wind velocity, the strongest being Category 5.