On the website:
You can favorite a food or recipe by clicking on the star icon to add it to your Favorites list. This can be done from the 3-dot menu button next to a food or recipe in your meal plan, or by clicking on the star icon when viewing a recipe or food details page.
You can find the recipes and foods you've added to Favorites on the site by clicking in the food search box in the left sidebar of the site, then clicking "Favorites" in the 3-dot menu button.
On the app:
The Favorites on the app are treated like a Collection. (The website will soon operate this way as well.) To add a food or recipe as a Favorite on the app, tap on the recipe/food to view its details page. Tap the star icon, then check off the box next to "Favorites" to add the food to your Favorites.
To view all of your Favorites, tap on "Preferences" at the bottom right of the screen, then scroll down and tap "Collections." From here, tap "My Collections" at the top, then tap "Favorites" to view all of the recipes and foods you've added to your Favorites list.