Warren Buffet is considered to be the most successful investor of the 20th century, and this quote is about success. He is saying that so long as very few of your decisions turn out to be mistakes, even just doing a few things right will bring you success.

It means that you don't have to do everything right in life to be successful. If you make a few right decisions (for example choose the right school, get the right job) your life will work out okay. However, if you make too many bad decisions then everything may not work out okay.

Buffett is saying all of these things at once: 1) If you do a LOT of things right and not too many things wrong you will be OK. 2) But you don't need to be that perfect. If you do just a FEW things right and not too many things wrong, you will be OK. 3) If you do TOO MANY things wrong, you will not be OK. Therefore, 4) If you are doing some things well enough, it is more important to concentrate on eliminating the bad things you are doing than on improving the good things you are doing. I don't know if Buffett meant this to apply to investing, or to life in general. Another investor, John C. Bogle, said something similar: "Successful investing involves doing just a few things right and avoiding serious mistakes."