Could mean either oif two things:
1) To get ‘tones’ of a spot colour ‘dots’ of the colour are printed with white space between the dots. This is commonly referred to as "half-tone screening" and occurs when separation files are made or when the image is printed. It is a function of the printer driver.
2) Spot inks are often used for "screen printing". Here the image is reproduced by ‘blocking out’ areas of a fine gauze printing screen by coating the gauze with a photo-sensitive emulsion that becomes water-soluable where un-exposed, and insoluable where exposed to light. The coated gauze is exposed to UV light through a film carrying the image. The gauze is then washed, removing the unexposed areas of the emulsion. The printing ink is then ‘squeegeed’ through the gauze onto the media.
Screening means (in offset lithography) applying a dot pattern to the color. The dot pattern can be percentages to minimal 5% to 100% solid color.
Talk to your printer about this to get line screen frequency based on the printing process that will be used.
Take notes and look for examples on the web by searching "Halftone Screen"
The actual screening will be done by the printer, so don’t worry about frequency, or angle or anything like that, it’s not your job.
From the designer’s point of view just consider "screening" to be making any color less than 100% printing (see the info palette).
