H-alpha (Ha, Hα, or H-α) is the first line in the Balmer series of hydrogen spectral lines. It has a wavelength of 656.3 nm (in air; 656.5 nm in a vacuum) and is red. Being visible light, it is conveniently within an atmospheric window. It is bright in the sky but does not stand out visually because it is so red as to be outside the most sensitive part of the eye's light sense.
An H-alpha filter (or hydrogen-alpha filter) is a narrow-bandwidth filter centered on the H-alpha wavelength. They have many uses, but are a basic tool in the study of the Sun.
Deuterium's H-alpha wavelength is about 0.2 nm shorter. With spectroscopy of sufficient spectral resolution, it produces a discernible feature on the H-alpha spectral line, enabling an estimation of the ratio between 1H and deuterium.