hi rose,
it is basically the patterns in which words (mostly nouns, verbs, adjectives) change to express little differences like tense (past, present...) or person (i, you, he...)
for example you see little bits of this in english, when you say, "i go, you go, he/she/it *goes*, ..." with -s added in the 3rd person singular. in german or icelandic, there are more different endings.
same with nouns. in english you can see it with pronouns, the little words like "i, you, he...", changing in different context, like "i see him" or "he sees me". but when it is a noun, it does not show change "the bird sees the fox" or "the fox sees the bird". but in a language like german or icelandic the nouns go into different forms (expressed through different endings and/or difficult articles in front of the noun) depending on the function of the word in the sentence. just like "i" changes to "me" or "he" changes to "him" in english.
understandable?
i know this is a bit weird to some native speakers of english at first (i taught german to english students as a summer job) but you should get a feeling for it very soon. learning the necessary patterns by heart and apllying them correctly is another thing, but manageable as well :-)
i'm struggling with icelandic in this regard right now myself. the grammatical rules are perfectly clear, as they work very much the same way as in german, but i have to learn all the patterns (like noun and verb endings) by heart, and that's hard work. but it's such a lovely language, so it is worth the effort! and, here at university they tell us: start modern icelandic first, and you'll get the basic understanding of old norse for free :-) the usual advice is to start old norse about on term later than modern icelandic, *not* to start both at the same time or old norse at first. i think this makes sense.
mona