1.
base (of a triangle or shape)
The bottom side of a geometric figure, especially a triangle. A standard term in mathematics education.
2.
bottom, lower strata, grassroots level
The lowest level or foundation of a society, organization, or hierarchy. Can refer neutrally to the grassroots or foundation, or negatively to the bottom rung of society.
Usage
Sense 1 is a standard math term learned in school. Sense 2 can be neutral (底辺を広げる, broaden the base/grassroots) or negative (社会の底辺, bottom of society). In recent internet slang, 底辺 is sometimes used pejoratively to describe low-status jobs or lifestyles, which many consider offensive.