Adjectives in Latin and English have degrees of comparison. In English we use "-er" or "more" to indicate the comparative degree which is used when two things are being compared: this apple is redder than that one; this book is more interesting. We use "-est" or "most" to compare more than two objects: this is the reddest apple I ever saw; this is the most interesting book.
In Latin, the comparative degree is usually formed by adding -ior, -ius to the stem of the adjective.
The superlative degree is usually formed by adding -issimus to the stem.