The Gunn-Peterson trough is a spectral feature in electromagnetic radiation from beyond about redshift 6, before the full effects of the epoch of reionization. It was predicted by James Gunn and Bruce Peterson in the 1960s and first observed around 40 years later when quasars at that distance were observed. It is a means of studying the universe at that time, e.g., identifying the redshift of the EOR and its progress.
The effect is a suppression of the spectrum at wavelengths longer than Lyman alpha (redshifted at the distance of the object), due to not-yet-ionized (neutral) hydrogen absorption by the intergalactic medium. That part of the spectrum is observed to be less than otherwise expected, e.g., weaker than that of more recent quasars.
Descriptions show the effect as a flattening of a spectral band with its longer-wavelength edge at the redshifted Lyman alpha wavelength. I haven't seen explanations of why it is more than a single narrow line, e.g., which other absorption lines contribute: it seems reasonable to me that the whole Lyman series could contribute, as well as thermal effects, and intervening neutral hydrogen at slightly smaller redshifts.