TypeScript is a language built on JavaScript that lets developers describe what kind of information their code expects. Those descriptions help tools catch some mismatches before the program runs.
For example, if a function expects a number but is given a date, TypeScript may warn the developer while they are writing the code. Think of it like labeling boxes so it is easier to notice when the wrong item is put inside.
TypeScript does not prevent every bug, and the browser usually runs the JavaScript produced from it. It is an extra checking aid, not a guarantee that the program is correct.