Briefly, it is an alternative display mode for multi-frequency records only. See the WFDB Programmer's Guide for details on multi-frequency records.
The -H option, introduced in WAVE 6.0, selects high-resolution mode. For ordinary records (those for which all signals are sampled at the same frequency), high-resolution mode is identical to standard mode.
In a multi-frequency record, some signals are sampled at multiples of the basic sampling rate. These are referred to as oversampled signals. In standard mode, WAVE decimates these signals (i.e., WAVE reduces their sampling rates to the basic sampling rate by averaging successive samples) before displaying them. In high-resolution mode, however, WAVE displays each sample from any oversampled signals if the screen resolution is high enough (note that at standard display scales, it may be difficult to see the difference). In this mode, WAVE replicates additional copies of each sample of any signals sampled at lower rates before displaying them; in consequence, particularly at highly magnified display scales, these signals may have a marked stairstep appearance.
Note that the time resolution for placement of annotations is the same in high-resolution mode and in standard mode. Thus, for example, it is not possible to place annotations on consecutive samples of an oversampled signal, since the second annotation will replace the first one. This is a limitation of the WFDB library, not of WAVE itself, and may be removed in a future release of the WFDB library.