The way to understand it it's almost synonymous with "barely" except it's not satisfactory.
"The food storage barely suffices for the winter" means with rationing meals people will pass the winter without starving but never full. It's not optimal, but it's acceptable.
"The food storage hardly suffices for the winter" means it will suffice to survive the winter but people will be starving. They won't die, but they will definitely feel strong hunger and lose weight. It's not something you should accept unless you have no other options.
"I barely passed the exam" - I needed score 40/100 and got 40 points. I can go have a drink and relax. It's not a stellar score but the job is done.
"I hardly passed the exam" - I needed score 40/100 and got 42 points. Technically, I passed. But the score counts towards global score which is required to advance to next year. Currently, with this exam I have 598/600 required and it means despite passing this exam I won't advance to the next year. I must approach this exam again and finish it with at least 44 points, then I will have barely passed into next year.
"Hardly" is sometimes an euphemism for "no". "That's hardly enough" usually means "do more, it's still not enough".