Everyone needs to eat. Some animals eat other animals, some eat plants and some eat both. Plants depend on sunlight, water, carbon dioxide and minerals/nutrients to feed themselves. Who eats who (or what) is called the food web.

A food web is a set of relationships between things that eat other things that eat other things.

Follow up:

Food web courtesy of USGS

Here are a couple games to play to help understand what food webs are.

If you have 2-3 people:

Have one child name a big animal (a predator, such as a bear or osprey, works best).
The next person names the first animal and something that animal eats. (eg: I am a bear and I eat fish)
The next person--or the first person, again--continues the list (I am a bear and I eat fish that eats salmonflies).
And so on.
I don't want to give away the ending, but eventually you'll get to plants that "eat" sunlight and water.

If you have a group of people:

Make name tags for everyone with animals and plants that can be found in a local ecosystem.
Stand in a circle with one child holding a ball of string.
That child says what they are (frog) and throws the ball of string (holding on to the end) to someone they eat or eats them (fly or bird).
The person who received the string holds onto it and throws the ball to someone they eat or eats them.
Eventually you'll have a web with everyone connected to everyone else. Just like in nature.
Then have one person drop their string and see what happens to the web. What happens when we take something out of nature?

After either game, look around or take a nature walk. Point out plants, animals or insects and try to figure out what they are connected to.

Do you have other ways to teach about food webs?

Books to teach about food webs in different ecosystems.

Arctic Appetizers: Studying Food Webs in the Arctic by Gwendolyn Hooks

Makers and Takers: Studying Food Webs in the Ocean by Gwendolyn Hooks

Freshwater Feeders: Studying Food Webs in Freshwater by Gwendolyn Hooks