Nonbasic is a card, usually a land, that doesn't have the basic supertype. As per Magic rules, only 4 of each nonbasic land can be in a deck at any one time. Basic lands do not have this restriction. Nonbasic lands often depict notable locations from the relevant set's style guide. 
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 205.4c Any land with the supertype “basic” is a basic land. Any land that doesn’t have this supertype is a nonbasic land, even if it has a basic land type.: Cards printed in sets prior to the Eighth Edition core set didn’t use the word “basic” to indicate a basic land. Cards from those sets with the following names are basic lands and have received errata in the Oracle card reference accordingly: Forest, Island, Mountain, Plains, Swamp, Snow-Covered Forest, Snow-Covered Island, Snow-Covered Mountain, Snow-Covered Plains, and Snow-Covered Swamp.
Note that a nonbasic land may have a basic land type (Plains, Island, etc).