So you're not far off the mark with how you interpreted what 'screening' means.

Yes, in short, screening a line means to hide the nature and strength of it behind a powerful picket, or series of outposts. A line can be screened in almost any circumstance, on the march, in a general advance, in defensive posture, in encampment, and so on and so forth.

For strategic purposes (such as on the march), the distance between the main body of an army and it screening units would be fairly large, several hours' or even days' march to give the main body ample time to react and prepare should their screen be pierced or 'pushed in.' The Napoleonic Corps system gave every Corps their own brigade of Light Cavalry to screen their front and tie into their neighboring corps. At most, the distance between Corps would be no more than a 24 hour march by lateral roads, to allow Corps to be mutually supporting in the event that one was struck. The disposition created the illusion of a spread-out army, but in reality was a powerful, mutually supporting and interlocked formation.

Cavalry, especially light cavalry such as Hussars, traditionally perform this form of far-ranging screening. An effective screen can completely mask the movement of an army and where it begins and ends (e.g: making it incredibly hard to flank). For example: In 1805 Napoleon used an entire Cavalry Corps in conjunction with another, regular Corps, to march towards the Black Forest while the rest of his force circled North. The thick, impenetrable screen of cavalry gave the impression that they were screening a large force, and the Austrians inability to penetrate that screen to a satisfactory degree made them fall for the bluff. Its a large factor of why Ulm was so successfully encircled.

Screening, however, can occur to the smallest tactical level. Even a more modern platoon on the march can screen and scout ahead of themselves by detaching picked scouts to travel ahead at great interval. In Napoleonic warfare, an Infantry division was almost invariably screened by a cloud of light infantry; such as Fusiliers, or Voltiguers or Riflemen. If the light infantry screen was effective, it could harass the enemy and cover their own forces, giving them time to conduct complicated evolutions (such as deploying from Column to line) unmolested.

Screening is both defensive and offensive in nature; it can allow for 'hasty attacks'; attacks of opportunity. In 1991, the US VII Corps screened its flank through the Iraqi desert with a handful of Armored Cavalry regiments (can you already see the basic premise has remained unchanged through the years?). The lead screens ran into an Iraqi armored unit still in laager (in camp) and conducted an attack of opportunity, the resulting confusion allowed main forces to approach and join the attack on the bewildered Iraqi forces. Likewise, screens advancing in front of an attacking unit may be able to push in weaker portions of the line on their own, creating sailents or finding holes, gaps and flanks in the enemy line that can be exploited by approaching forces with greater manpower and firepower.

Screening is considered the realm of reconnaissance units, but any unit can ostensibly do it. Some scholars describe Patton's use of his attached tactical air corps as an example of 'air screening' when he used it en masse to cover his Southern shoulder. The key to screening is presence and mobility without the burden of manpower. Almost every unit, from the Army to Battalion level, have a proportionate scouting element assigned to it; a rifle battalion almost always has in modern examples, at least a Platoon of mobile troops (to a relative degree) able to scout and screen for it.