Resolution is pixel count: 1920 by 1080 remains the working standard of club visuals, 3840 by 2160 (4K) the premium tier, and LED walls whatever their panel grid multiplies out to, often odd sizes like 1728 by 576.
Render cost scales with pixels, so resolution is the single biggest lever on the frame budget: 4K is four times the fragment work of 1080p. Performance systems often render below output resolution and upscale, spending the savings on effects and frame rate, a trade audiences rarely detect in motion.
The practical rule for gigs: ask what the wall or projector actually expects and match it, since scalers in the signal chain add latency and soften the image.