A hex bolt is a metric (or imperial) machine bolt with a hexagonal head that takes a spanner or hex socket. The thread can run the full length of the shank (fully threaded) or stop short and leave a smooth shank under the head (partially threaded). Partially threaded versions are slightly stronger in shear because the unthreaded portion has more cross-sectional area.
Gate-trade sizes run M8, M10, M12, and M16, in grade 4.6 (mild) or grade 8.8 (high tensile). Galvanised hex bolts are stocked everywhere; stainless 304 or 316 hex bolts are needed for marine exposure.
For structural connections (footplate to post, hinge plate to wall, anchor plates to concrete), hex bolts are the default. Match grade and length to the calculated load and embedment depth.