what 'inflected' means and why it is different to English?
If words are inflected their endings change according to case, person, tense, etc. In Polish there are 7 cases and 5 persons that influence how nouns and adjectives look like i.e. they require applying appropriate endings to them. Similarily 3 tenses and things like aspect and mood influence verbs (their endings and also used prefixes).
This subject is very well explained in Wikipedia.