Orthogonal frequency-division multiplexing (OFDM) is a technique used for which types of amateur communication?
Why OFDM splits a data stream across many closely spaced subcarriers whose spacing is chosen so each one's spectrum nulls fall on its neighbors' centers, meaning they do not interfere despite overlapping. Each subcarrier carries a slow, low-rate portion of the data, which makes the signal very resistant to multipath and selective fading. In amateur service it shows up in high-throughput digital modes and is the basis of Wi-Fi, DSL and digital broadcast systems.
Watch out OFDM is perfectly legal on the amateur bands as long as the emission fits the band's rules, so the choice claiming it is prohibited is simply wrong. It is also not tied to weak-signal work like EME or QRP, where narrow single-carrier modes dominate.
OFDM = many parallel subcarriers = data. Think Wi-Fi and DSL: it is a digital mode technique.