This helped me with that question. There are quite a few adjectives in french for which position is important for their meaning.
The link to french.about.com is helpful, but in this case, I think it's also a matter of idiomatic usage. I suppose the closest translation to "mes grands amis" would be "my good friends." If I said "These are my good friends" in English, you would understand that I am not referring to the friends' ethical qualities or to their friendship skills (the way that "a good soccer player" is skilled at playing soccer), but to the fact that I am particularly close to them. Likewise, in French, "mes grands amis" is normally understood to refer to how close I feel to the friends, not to their height, size, or greatness.
This is a rule you will have to apply VERY often on Duolingo.
In French, "c'est" (sing.) and "ce sont" (plural) are used in a large variety of expressions, when a pronoun (it, she, he, they) is subject of verb "être" and followed by a nominal group, ie: article (+ adjective) + noun.
- it is + noun => c'est + article + noun
- she is + noun => c'est + article + noun
- he is + noun => c'est + article + noun
- they are + noun => ce sont + article + noun
"ces" is a demonstrative adjective : ces amis sont les miens (these friends are mine), plural of "ce", "cet", "cette". It agrees with the following noun:
- ce chien - cet éléphant - cette chienne - ces animaux
"ce" is a pronoun: "c'est" (elision/apostrophe in front of a word starting with a vowel) or "ce sont". It is the subject of verb "être".