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Line blanketing (or the blanketing effect, or line-blanketing effect) refers to an effect of many simultaneous absorption lines, but I've seen two explanations of the term "blanketing". One explanation is that a large number of such lines covering an entire region of the spectrum can completely hide that portion of the continuum (like a blanket hides what is underneath). The other explanation is such that a bunch of absorption lines can act as insulation (like a blanket): each line is absorbing some the energy that would otherwise pass through, and subsequent spontaneous emission is in all directions, sending some energy back to the EMR source that produced the continuum, trapping some of its heat, resulting in a higher temperature. These two phenomena generally occur together, e.g., in stars.
A high density of absorption lines can result in spectrograph data showing a region of wavelengths all with less than the continuum. This can be due to a low spectral resolution (i.e., a higher spectral resolution could show the lines) but also can be because the line broadening is sufficient that there are no gaps between the lines irrespective of the instrument's spectral resolution.
Line blanketing is commonly the result of metals (with their additional structure producing a larger variety of lines), and stars with more metals display more. The net effect is an absorption band on an underlying black-body spectrum for a slightly higher temperature than would otherwise be the case. Its presence can be detected by spectroscopy and often by color indices, the latter allowing photometry to estimate stars' metallicities. Molecules similarly increase blanketing.
The opacity of Earth atmosphere to some wavelengths (resulting in atmospheric windows) is due to line blanketing (in the first sense, blocking an entire wavelength-interval), e.g., in the infrared. The greenhouse effect is the blanketing effect contributing to a planet's temperature.