Collecting light
A telescope's most important property is its aperture — the diameter of its main lens or mirror. Light-gathering power grows with area: Webb's 6.5 m mirror collects about 6 times more light than Hubble's 2.4 m mirror.
Resolution
The finest detail a telescope can resolve is limited by diffraction: θ ≈ 1.22 λ/D. Bigger apertures and shorter wavelengths resolve finer detail. On the ground, atmospheric turbulence blurs images, which adaptive optics can partly correct.
Why space?
Earth's atmosphere absorbs most infrared, ultraviolet and X-ray light and blurs visible images. Space telescopes avoid both problems. Webb also needs extreme cold — its sunshield keeps the optics below about 50 K so their own heat does not swamp faint infrared signals.