The workhorse of brushes. Flats are great for laying in big blocks of color but can also be turned to make very thin lines when loaded and sharpened with paint.
Brights are like shorter flats. I don’t buy brights because eventually flats become brights from being worn down. Brights are great for scumbling and scrubbing paint. Their shorter bristles push paint rather than laying it down.
Filberts are like flats with rounded sides. The filbert is a good brush to reach for when you don’t want the sharp edge of a flat. There are also variations on the filbert. One of these, and I’m not making this up, is called an Egbert. This extra long filbert will hold a lot of paint, or when used dry, it’s a good blender brush. I have a few that I use primarily for drawing with thin inky paint.
There’s no roundabout way to say it, so I’ll get right to the point. You might think you need a really small round brush for tight detail. Go for the bigger brush that’ll come to a sharp point though. That big brush can hold more paint in its belly and you’ll be going back to the palette less often so your stroke can be continuous.