Let’s define a complete protein first. A complete protein is a protein source which contains a good dose of all 9 of the essential amino acids. These 9 amino acids are vital for our bodies to grow and to repair on a regular basis. Most veggies aren't complete proteins on their own as they normally only contain a few amino acids.
However, if you include a big variety of veggies, grains and legumes in your diet, you’ll no doubt cover off all the essential amino acids throughout the day. On the other hand, meat always contains all the 9 essential amino acids, which makes it a complete protein.