Yeasts eats simple sugars and needs help to eat the big/complex sugars in Barley. Grain holds some of its sugars in polymers – starch and cellulose. Polymers are built up structures of units – in this case we are talking about structures of sugars (polymer can create plastics. It is a construction)
Yeast can’t digest starch. In malting we create the conditions for the grains to break down it’s sugar polymers ie starch – so the yeast can eat them. We do this by making the barley grain think it is time for it to grow. So it naturally starts breaking down the polymers into energies it can use to grow. Just at the right time, we heat/dry the barley and this stops the growth, so, the sugars we need for the yeast to eat have been naturally broken down by the grain itself.
Sugars are carbohydrate which means their construction is carbon atoms with hydrogen and oxygen atoms attached. Things are things because of the pattern they are constructed in. The same components in a different structure give us something else. Starch and cellulose are made from the same components, but are different things due to their structure.