得ない is a compound suffix that combines with the ます-stem of a verb to form a negative potential expression. It means 'cannot' or 'unable to', but it is stronger and more formal than the standard potential negative (e.g., できない). It is often used in written language, formal speeches, or serious statements. For example, あり得ない (arienai) means 'cannot exist' or 'impossible', and is even used in casual speech as a strong expression of disbelief (like 'No way!'). However, most other forms sound stiff and literary, so you should avoid using them in everyday conversation unless you want to sound dramatic or formal. When you want to say something is impossible in a normal way, use ～ことができない or the potential negative form of the verb (e.g., 言えない).