How far the blade steel runs into the handle. A full tang runs the whole length and close to the full width of the handle, with scales bolted to either side and steel usually visible around the edge. A partial tang stops short of the butt, saving weight and cost at the price of leverage strength.
Look at the handle side-on. If bare metal runs along the top and bottom of the grip and shows at the butt, the tang is full. If the handle is unbroken material all the way round, it is partial or hidden.
Hidden and partial are not synonyms: a hidden tang can run the full length inside a one-piece handle. Whoever is after the other pieces will find them in the parts of a fixed blade.
A full tang has no point along the grip where the cross-section suddenly drops, so there is no obvious place to break under sideways load. That matters for batoning and prying.
A partial tang is perfectly adequate for cutting. Its weak point is impact and leverage, at the transition where steel gives way to handle material.
Full tang for camp knives, hunting knives and anything you might lever. Partial tang for fillet knives, kitchen knives and light utility blades, where the weight saving is a genuine benefit.
Full tang is not automatically better. It is better at one specific thing.