Techniques described as meditation include practices that are almost opposite in method. The useful division is between narrowing attention to a single object and opening it to whatever appears.
Concentration practice restricts the field deliberately
In focused practice a single object is selected, commonly the breath, a sound or a repeated phrase, and attention returns to it whenever it departs.
Everything else is treated as a distraction, including thoughts that are interesting or feelings that are pleasant.
The narrowing is the technique, and with practice it produces a stable, absorbed state that practitioners describe as collected rather than relaxed.
Open monitoring removes the object
The alternative approach involves attending to whatever arises without selecting anything in particular, noticing sounds, sensations and thoughts as they appear and pass.
Nothing is a distraction, since there is no object to be distracted from, and the instruction concerns the manner of attending rather than its target.
The state is typically described as spacious and alert, and it is more difficult to sustain because it lacks the anchor that focused practice supplies.
They are usually taught in sequence
Most traditions teach concentration first, on the grounds that open awareness without a degree of stability collapses into ordinary distracted thought.
Focused practice builds the capacity to notice where attention has gone, which is the prerequisite for open practice to be anything other than daydreaming.
This is why beginners given open instructions without groundwork often report that nothing is happening, which is an accurate description of what usually occurs.
The states differ in what they do afterwards
Concentration practice tends to be described as calming and is generally the form associated with reduced physiological arousal during the session.
Open monitoring is more often associated with changes in how experience is related to, including the sense that thoughts are events rather than instructions.
Research examining the two separately has found differing patterns of brain activity, though interpretation of those findings remains an area of active work.
Neither is a superior technique
They address different aspects of the same training, and traditions that emphasise one still use the other in some form.
Which suits a person at a given time depends on the state they are in, since somebody highly agitated usually finds an anchor easier than an open field.
Anyone finding that meditation increases distress rather than reducing it should stop and discuss it with a teacher or clinician, as this is a recognised occurrence rather than a sign of doing it incorrectly.