A gradable adjective is a word that describes a quality that can be present in varying amounts, such as "old" or "fragile." In contrast, a non-gradable adjective, describes a condition that is either true or false, with no middle ground, such as "married" or "former." One person might be described as more or less old than another person, but it would not make sense to say that one person is less married than another.
In general, a gradable adjective can be modified with adverbs relating to amount, such as "very" or "somewhat," and can have comparative and superlative forms. For instance, something can be "very sad" or "somewhat sad." The word "sad" also has a comparative form — one story can be sadder than another. It has a superlative form as well — a third story can be the saddest of all.
Gradable adjectives can be characteristics that are either qualitative — able to be measured — or quantitative — unable to be measured. For example, "hot" is a gradable adjective because it describes a condition that can be more true or less true. A day with a high of 100°F (37°C) is hotter than a day with a high of 90°F (32°C). Gradable adjectives can also refer to something that cannot be objectively measured, such as "interesting." The same book can be considered more interesting or less interesting depending on who is reading it.
A native English speaker can usually determine whether an adjective is gradable or non-gradable by considering if it makes sense with the word "very" used before it. Non-native speakers may find this a bit more challenging, since a number of adjectives seem like they should be gradable, but actually are non-gradable. The word "superb," for example, simply means "really good," so it seems like something could be more or less superb. Grammatically, however, "superb" is actually non-gradable.
Some words can with multiple meanings gradable or non-gradable depending on their context. For instance, "liquid" has a scientific meaning that is non-gradable. A substance that is liquid is in a particular state of matter. On the other hand, in non-scientific usage, a soup with a very thin broth might be considered more liquid than one with a thick broth.
There are also a number of adjectives that most grammarians would consider non-gradable, but are often treated as gradable in common usage. For instance, the word "unique" technically means "the only one of its kind." By this definition, to say that someone has a "unique voice" would mean that not a single other person in the world has a voice like that person's. Often, however, "unique" is used as a gradable adjective meaning "unusual" or "uncommon," as in, "He has a very unique style." The latter usage is considered incorrect in formal speech or writing.