The class of “carbohydrates” covers a large variety of related molecules that are made from building blocks called “monosaccharides”. There are simple carbohydrates that are made of a few of these building blocks, and more complex carbohydrates that consist of long complex chains of these blocks.

Sometimes “sugar” is used a synonym of “carbohydrate” - but it is also common for “sugar” to be used to mean more specifically the molecule “sucrose”, which is a very simple carbohydrate consisting of a glucose block and a glucose block.

These carbohydrates can be chopped up into the basic blocks, and these blocks are then absorbed into our blood. If you eat very simple carbohydrates containing glucose - such as sucrose, then the enzymes in the intestine can split it very quickly and the glucose level in your blood will spike rapidly.

If, on the other hand, you eat more complex carbohydrates, the breakdown of these will be slower and they will be released into the blood over a longer period of time.

If one person eats pure ‘sugar’ (sucrose) and the other person eats the same amount of carbohydrates but in the form of whole grains, the person that eats pure sugar will have a sharp spike in their glucose level which will then decrease, whereas the person that ate whole grains will have a slow release of those carbs over the next few hours.

This is a simplification that ignores the complexities of metabolic regulation and the many cases in which it goes wrong (for example, diabetes), but it is a rough picture of what it means.

So you would not want sugary cereals, as these will give you simple sugars that will cause a spike. You want to eat less-processed whole grain meals and legumes, which will provide you with more complex carbohydrates that will be released slowly over time.

The term that describes the rate at which carbohydrate-containing foods increase the glucose levels in the blood is the glycemic index, and you can find some examples here: https://en.wikipedia.org/wiki/Glycemic_index