A possessive adjective is something that is used in French to show WHO something or someone belongs to. In English the equivalent would be 'my', 'your'. So for example:
'My' bag. ('My' is the possessive adjective in this case).
In French, this would be 'mon' sac.
However, in French it gets a little more complicated, because of the fact that their nouns (aka their words which describe things, or people), are either MASCULINE, FEMININE or PLURAL.
This means that when you use a possessive adjective, it has to AGREE with the noun you are describing.
So 'mon' sac uses the word 'mon', because 'sac' is MASCULINE.
In contrast, the word 'trousse' (pencilcase) is FEMININE. Therefore, if you wanted to say: 'my pencilcase,' you would have to use the feminine version of 'my', which is MA.
Therefore, 'my pencilcase' becomes 'ma trousse.'
Another example of PLURAL use of a possessive adjective would be the following:
If you wanted to describe:
'my sweets', because the word sweets is PLURAL (aka, you have MULTIPLE SWEETS), you would have to use the possessive adjective 'MES.'
Therefore, 'my sweets' would become 'mes bonbons.'
Take a look at this gcse bitesize table to help you out with all the different forms fo possessive adjectives! http://www.bbc.co.uk/schools/gcsebitesize/french/grammar/possessivead/possesiveadjectiverev1.shtml