Welcome to KIC and apparently the coaster community!
Like what Kenban said a giga is the name given to a roller coaster that exceeds 300ft, similar to how a hyper coaster is a coaster that exceeds 200ft. Gigas are known for their height and speed. Most gigas are low to the ground after the first initial drop, though Steel Dragon 2000 has plenty of tall airtime hills. Examples of gigas would be Millenium Force, Intimidator 305, Leviathan, and Fury 325 to name a few. Gigas are quite rare in the coaster world compared to hyper coasters or even inverts though Cedar Fair has cranked out 3 in the past 7 years. Kings Island is the last main Cedar Fair park that hasn't gotten a giga coaster yet.
Here's two examples of giga coasters made by two completely different companies
Fury 325 made by Bolliger & Mabillard
Millennium Force made by Intamin