Substantives are words that are used to mean things, whether concrete or abstract. Words that are listed in dictionaries as nouns or adjectives, because those are their main uses, can both be used as substantives. So words like 'British', which certainly look like adjectives, and which are given in dictionaries as being primarily adjectives, can be used after 'the' - "the British drink tea" for example - and are then obviously behaving like nouns. It is sometimes convenient to use the word substantive to describe a word like this, without arguing about whether it is a noun or an adjective.
(The opposite - a word that describes a noun while being in itself either a noun or an adjective - can be called an epithet.)
- You may also like to see AWE's page distinguishing between substantial and substantive.