The term complete protein means that a food contains a combination of all 9 essential amino acids (the building blocks of protein). The essential variety cannot be made by the body therefore must be obtained through diet. Protein takes longer to digest than carbs and fat, prompting your body to release Leptin (correlated with feelings of fullness) and lower levels of Ghrelin (a hunger inducing hormone). This is why proteins make you feel full for hours.
You can create a complete protein by combining plant-based foods. For example, beans and nuts tend to complement each other and complete the amino acid profile. We use NutritionData from Self Magazine to research and build high-protein meals for clients.
Check out this example nutrition visualization of walnuts. Note that under “protein quality” you can select to “find foods with complementary profile.” This option is available for every food. Try it the next time you’re building a salad and notice the difference!