Transiting Exoplanet Survey Satellite (TESS) is a spacecraft intended to survey 2,000,000 nearby stars for transiting planets, launched in April 1918 for a 2-year primary mission. It uses four wide-angle telescopes and stores three months' data, allowing it to send down only promising examples. It aims for stars brighter than 12 magnitudes, G-type stars and K-type stars, as well as nearby M dwarves, small M-type stars.
TOI stands for TESS object of interest, a term for a possible planet, i.e., the location of observations that appear to reveal a transiting planet. TFOP (for TESS Follow-up Observation Program) is an effort to investigate TESS discoveries, e.g., to confirm whether a TOI is a planet.
As of 4/2019, TESS has produced over 500 candidates (TOIs), including 100 apparently between Earth and Neptune in radius, and 9 confirmed through follow-up observation to be planets.