If the sampling rate that the file is recorded at is 88.2 kHz, you get 1 sample every 1/88,200th of a second.
Now if you take that file and play it back at 44.1 kHz, without resampling it, you get 1 sample every 1/44,100th of a second. It takes twice as long to play each sample than it did to record it. The result is that it takes twice as long to play the file back, which what happens to sound recorded on a tape when it's played back at half-speed.
You can prove it to yourself, just try it. Sound Forge has a check box on the Resample dialog box labeled Change sample rate only (do not resample). I'm sure most audio editors allow you to do this; it might be called something different, but there must be a way.