Answer:
A "comparative" probably refers to a "comparative adjective".
An adjective describes a noun. (A noun is a person, place, thing or idea.) In other words, it gives information about something or someone.
That blue bag weighs 20 kg. It is heavy > 'heavy' is an adjective which gives info about bag.
A comparative adjective compares two or more nouns to give information about which one has (1) more (or less) or (2) the same of a certain quality or quantity.
(1) MORE OR LESS ----------
The blue bag weighs 20 kg. The red bag weighs 40 kg.
The blue bag is heavier than the red bag, or
The red bag is lighter than the blue bag.
In English, the comparative of an adjective is formed:
(a) If the word is short (i.e., it has only one syllable), add the ending -er
HARD -> HARDER
(b) If the word is short and ends in the spelling pattern VOWEL (a,e,i,o,u) and a CONSONANT, double the final consonant and add -er
BIG -> BIGGER
(c) If the word ends in -y, change the -y to -i and and -er
HAPPY -> HAPPIER
(d) If the word is longer (i.e., it has two or more syllables), add the word more (or less) before the adjective
INTERESTING -> MORE (or LESS) INTERESTING
(e) Irregular forms. Some adjectives have irregular forms. Here is an incomplete list:
good -> better
bad -> worse
far -> farther or further
(2) THE SAME ----------
If the two things have the same quality or quantity, use the words "as ... as".
The green and yellow bags both weigh 50 kg. The green bag is as heavy as the yellow bag.
ADDENDUM ----------
If there are more than two objects, the superlative form is possible, however, that is beyond the scope of this question.