It's exactly what it says on the tin.
A whole food plant based diet is one that uses food in its whole form and only from plants which means not eating any animal products such as meat, fish dairy or eggs. The term implies that you eat a minimum of processed foods so avoiding excess oil, salt and sugar.
People often think vegan and plant based is the same thing but there is a difference. If you are following a whole foods plant based diet then this is usually driven by a desire to have a healthier body. Whereas a vegan will have moral and ethical stances on the way animals are treated and used for human’s purpose. For example a vegan wouldn’t wear any animal skin, eat animal products, go to zoo’s and marine parks, they don’t own or use animals for their own purposes such as riding horses or working dogs, they purchase cruelty free cleaning and personal products such as skin care and make up, they would make ethical and environmental decisions on purchases and ways of living and they will only donate to medical charities if they do not test on animals.
Although a vegan diet also avoids animal product the main difference is that you can be a processed junk food vegan and never see any improvements in your health or longevity.