Monocolored means one color in respects to a card's or deck's color (i.e. white, blue, black, red or green).
From the glossary of the Comprehensive Rules (September 25, 2020—Zendikar Rising)
- An object with exactly one color is monocolored. Colorless objects aren’t monocolored. See rule 105, “Colors,” and rule 202, “Mana Cost and Color.”