The tang is the part of the blade steel that continues back into the handle. It is one continuous piece of metal with the blade, not a separate part joined on, and how far it runs is the single biggest clue to how much abuse a fixed blade will take.
Take any fixed blade apart and you find one piece of steel running from the tip to somewhere inside the handle. Everything forward of the guard or the handle scales is the blade. Everything behind it is the tang. The pieces around it are walked through in the parts of a fixed blade. On a folding knife the same word is used for the rear portion of the blade that pivots inside the frame, which is where a slip-joint’s backspring rides and where traditional makers stamp their name.
Because the tang is the same bar of steel as the edge, it never has a weld or a joint to fail. What varies is how much of it there is, and that is the whole reason the word appears on spec sheets at all.
A knife whose tang runs the full length and width of the handle has no thin section to snap when the blade is levered sideways. A knife whose tang narrows to a rod, or stops a third of the way in, is lighter and often cheaper to build, and it is entirely adequate for cutting. It just has less margin if the knife gets used as a pry bar.
Read the spec line before you read the price. Full tang, partial tang and hidden tang describe three genuinely different amounts of steel, and manufacturers state which one they used.