A food chain describes a chain of feeding in a system - or the way in which energy moves between different plants and animals. This means a chain may describe what a salmon eats and what eats it!
An example of a food chain could be:
The sun shines on the grass, which makes it grow. A cow eats the grass, which it needs for it to be able to produce milk. Humans can then drink the milk that comes from the cow.
An example of a food chain involving salmon could be:
The sun shines on the water and plankton use the light from the sun as energy. Plankton are present in water and these are eaten by small invertebrates such as fresh water shrimps (called gammarus).
The young salmon eat the fresh water shrimps and other small invertebrates. Larger fish such as trout or even pike (see threats) can feed on the young salmon. When the smolts head off to sea they travel great distances to their feeding grounds where they grow large. Adult salmon can then be eaten by humans.