Fewer refers to things that are countable.
We had fewer people at the fundraiser than we had hoped.
Fewer tornadoes occurred this year than last year.
Generally, less refers to things that are not countable.
Sue has less concern for her dog’s safety now that the backyard fence is completed.
Less talking would help my concentration.
However, the expression less than is used in front of a plural noun that denotes a measure of distance, amount, or time.
We will go on vacation in less than four weeks.
She owes him less than $30.
We had less than 25 miles to go but ran out of gas.