Structured Illumination (SI) - or patterned illumination - is the wide-field version of breaking the diffraction limit of light. SI relies on both specific microscopy protocols and extensive software analysis post-exposure.
The main concept of SI is to illuminate a sample with patterned light and increase the resolution by measuring the fringes in the Moiré pattern (from the interference of the illumination pattern and the sample). "Otherwise-unobservable sample information can be deduced from the fringes and computationally restored." (Gustafsson MG, PNAS 2005).
SI enhances spatial resolution by collecting information from frequency space outside the observable region. This process is done in reciprocal space (Frequency, or Fourier Space).