Move the pin in each bottom floor bracket to adjust the vertical gap and the space between the bottom of the doors and the floor. There are two types of bottom brackets. One requires you to loosen a set screw and slide the bracket in or out. The style shown in the photo requires you to lift the door and drop the pin into a different notch in the bracket.
Like a car that needs new spark plugs, bifold doors tend to run a little ragged after a few hundred uses. They stick, they rub, they don't line up. Luckily they're easy to tune up once you know the tricks.
The goal is to adjust the doors to leave an even 1/8-in. vertical gap between them (Photos 1 and 2) and an even horizontal space along the top—without allowing them to scrape on the door jambs or floor. You can't always get the doors perfectly aligned on all four sides, especially if the opening is out of square. If you encounter this situation, get the highly visible gaps between the doors and the top even (Photo 3), and leave uneven spaces either where the doors meet the side jambs or at the floor. Once you get your doors fine-tuned, spray a little silicone lubricant on the track for a really smooth glide.