Computer line-in voltmeter
Why does it work only with alternating current?


Line-in is for audio signals. A DC signal represents silence, whether it is 0V 0.5V or 1V. Therefore the desgners of the Line-in circuits are not interested in DC levels at all.

As pjc50 commented, technically, DC is typically filtered out using a series capacitor.


  What information does captured wave contain?


Line-in is designed to be used to accept varying voltage levelswhich represent variations in air pressure over time. Typically such variations caused by spoken conversation or music near to a microphone (or indirectly from a recorder, or synthesised versions of this). 

So long as the analogue signal is sampled at a high-enough frequency, the stored values can later be used to reproduce a good approximation of the analogue signal sufficient to potentially reproduce the variations in air pressure over time (using an amplified and loudspeaker/headphones/earphone).