Upsampling is the process of inserting zero-valued samples between original samples to increase the sampling rate. (This is sometimes called “zero-stuffing”.) This kind of upsampling adds undesired spectral images to the original signal, which are centered on multiples of the original sampling rate. “Interpolation“, in the DSP sense, of the process follows the upsampling by filtering. (The filtering removes the undesired spectral images.) As a linear process, the DSP sense of interpolation is somewhat different from the “math” sense of interpolation, but the result is conceptually similar: to create “in-between” samples from the original samples. The result is as if you had just originally sampled your signal at the higher rate. Upsampling DAC manufacturers claim that their products improve the sound quality of standard CDs as compared to conventional DACs and most listeners agree. Upsampling can also take place inside of DAW systems at various points. For instance, some plug-ins are based on processes that work better at higher sample rates. Part of the plug-in’s software code will include an upsampling process that occurs before the plug-in acts on the signal, and then downsample’s the audio to the sample rate of the session to re-inject the signal back into the main audio path.