"For seven days you are to eat matzah—on the first day remove the leaven from your houses. For whoever eats hametz [leavened bread] from the first to the seventh day is to be cut off from Israel."
This is where the term 'Kosher for Passover' comes from. While it is fine to eat hametz the rest of the year, it is forbidden during the Feast of Unleavened Bread.
Hametz or chametz is often translated as "leaven" but that is misleading. Yeast should not be eaten during Passover but that's not explicitly what hametz refers to. Hametz refers to grains that have come into contact with water. It is related to chometz, meaning vinegar, and chamutz meaning sour. Meaning fermented grain products should not be consumed during the Feast of Matzah.
What are these grains? Scripture doesn't specify, though wheat is the best guess. I would encourage you to study and pray about the grains to avoid. Traditional Judaism avoids:
Please note that since it is the Feast of Unleavened Bread, unleavened bread is still perfectly okay to consume! Traditional matzah is made from wheat. You can purchase it Kosher for Passover or make your own.
An interesting hametz product that doesn't immediately jump to mind is vinegar. Vinegar is distilled from wheat that has obviously fermented and become hametz. This makes it's way into lots of condiments that should be avoided during Matzah Week. Unless a vinegar is specified as a wine vinegar or apple cider vinegar, consider it hametz.
So what all should be avoided during the Feast of Unleavened Bread?
Bread, croissants, biscuits, etc.
Pasta and noodles
Breakfast cereals such as Cheerios, Shredded Wheat, Raisin Bran, etc.
Baked goods with flour such as cookies, muffins, cake, etc.
Condiments with vinegar such as ketchup, mustard, mayonnaise, salad dressings, salsa, soy sauce, and pickled vegetables.
Barley - soup or risotto
Oats and oatmeal
Also watch out for oatmeal in soaps and bath products.
If this is your first year keeping Kosher for Passover, this can sound a little overwhelming, but hang in there! There are still plenty of things you can eat.
What can we eat during the Feast of Unleavened Bread?
Any fresh vegetables
Any fresh fruits
Beans - black, pinto, Cannelini, kidney, chickpeas, etc.
Winter squash - butternut, acorn, spaghetti, etc.
Any kosher meat or fish
Dairy - yogurt, cheese, milk, butter, sour cream, etc.
Salad dressings made with balsamic or apple cider vinegar
Gluten free soy sauce, called tamari
Desserts like cheesecake, creme brulee, flourless chocolate cake, and macarons
Beef or chicken fajitas with corn tortillas, black beans, guacamole, and onions and bell peppers
Stir fried broccoli, carrots, cabbage, peppers, etc. over fried rice with tamari
Baked salmon with roasted butternut squash and spinach salad with balsamic vinaigrette
Scrambled eggs with mushrooms and goat cheese served with matzah
Baked potatoes with broccoli and cheese
Spaghetti squash with marinara sauce
Coconut curried vegetables over lentils
Matzah spread with cream cheese, topped with smoked salmon, arugula, and cucumber slices
Lettuce salad topped with chicken, hard-boiled eggs, nuts, carrots, tomatoes, etc. and salad dressing from apple cider vinegar
Lots of soups: black bean, vegetable, lentil, potato cheese, etc. served with matzah
Apples with peanut butter
Hummus with veggies and matzah
Yogurt with fruit or jam
Trail mix with mixed nuts, dried fruit, and chocolate chips
Here's more than 40 recipes that are Kosher for Passover.
Exodus 12:15-20 explains that the Festival of Matzah is an ongoing instruction that should be celebrated by every generation of YHWH's people. Matzah doesn't have to be the only thing we eat, but it does say that we should eat it so have some each day and make it a central part of the Feast!
Have a wonderful Feast of Unleavened Bread!