Sometimes people suppose that prayer is meditation - but although there may be some overlap between prayer and meditation - and the two may alternate in the same 'session' - it may be helpful to regard the two as different.
I have used the analogy that prayer is a conversation with God, while meditation is being with God; that meditation
is therefore a communion - and what comes from it is not so much
information or guidance; but instead things more like motivation,
sweetness, love, companionship, encouragement, inner strength.
But the most important clue to Christian meditation is that:
Meditation is inwardly directed, while prayer is externally-directed.
(But the two states are not symmetrical - inward direction is more like inward awareness, than it is like 'looking' within.)
Christian meditation is possible because God is within us.
This fact can be conceptualized in various ways - as a divine spark, perhaps - the metaphor which makes most sense to me is that God is within us like a glowing coal (a potential source of energy, warmth and light).
(That God is within us in a literal sense, however conceptualized, is a necessary but mostly indirect, inference from scriptural revelations such as that we are children of God, that our destiny is to become Sons of God, that we are able to make good choices - including the choice to follow Christ, that we are not wholly evil and so on.).
Christian meditation is therefore an act of communion with God within us.
Meditation is not essentially a focused, but a receptive state.
Meditation is not a purposive act but rather intends to allow our selves to be suffused with divine influence.
Meditation is not an alert state, but an attuned state - we hope to harmonize with the divine, we aim at empathic identification with the divine.
This last point concerning empathic identification can be elaborated: empathy is sympathy, which is to resonate with the other. By empathic identification with God we briefly and partially share in His nature, hopes, purposes - we may gain clarification about our proper orientation in life.
All the above emphasizes that meditation is a trance-like, dreamlike state of altered consciousness - not alert, nor active, nor problem-solving - so to meditate requires a situation where alertness, activity and problems are not required, are set aside. And since the attention is inward (not focused inward, but allowing what is within to suffuse the self) then the situation for meditation must be somewhat removed from external stimulus, from anything which would tend to direct attention and interest outside the self - these are distractions to meditation.
The essence of meditation is therefore not a matter of the external senses and perceptions (such as vision and hearing) but primarily a matter of the sixth sense - the inward directed perception of body states, of emotions.
This is the reason why meditative states are hard to describe - because the private and inward information which comes from meditation must first be translated into the public language of vision and perhaps hearing - e.g. described as pictures and voices. These are second order descriptions, and only as accurate as would be any description of what an emotion feels like.
What makes meditation Christian is that a Christian knows that God is within us, that God is good, that he is a child of God, that Christ is our Lord and Saviour and so forth. Such faith frames and controls the meditation - lacking Christian faith then inward-attentive meditation may only find other things that are not God - for example our own instincts and impulses, information and perspectives we have learned or been inculcated-with, or evil spirits and influences.
So meditation per se is a neutral activity - what makes meditation good or bad is the purpose of the meditation (the real true purpose), and the object of the meditation (at what it is directed) - whether God or something-else.