Most people who advocate 'meditation' have in mind something which is undirected, often deliberately anti-purposive, often passive and responsive - and there may well be a training in such meditation with exercises, instructions, breathing practices etc.
(The current fad for 'mindfulness', for example, comes under this heading.)
After expending quite considerable time and effort on such things, observing the effect on people who have expended even more time and effort on such things, and on theoretical grounds - I believe this type of meditation does not go anywhere, does not lead to positive personal transformation, and is not helpful in our primary life task of theosis (i.e. becoming more divine)
Undirected meditation may, therefore, be 'therapeutic' in the sense of making someone feel better or perhaps even function better; but it does not of itself lead to genuine spiritual progression - and may indeed stand in the path of it.
Meditation is a means to an end, which implies that the end is more important than the means - and you need to know where you are aiming before you set-out: including that we must have a goal before we meditate, each meditation should be driven by some real and personally-compelling purpose and need, and meditation should be honed and developed according to how well those goals are being reached.
Useful meditation therefore requires a valid metaphysical understanding of reality - in other words some approximation of Christian knowledge. Then meditation can be deployed for Christian goals - eg. (and these are only examples) as a kind of prayer to seek understanding or to clarify existing understanding, to seek revelations, to commune with the divine, to seek for strength and courage, to alleviate incipient despair and become re-infused with hope... that kind of thing.
So, when meditating, if it is to be more than merely some kind of self-induced 'trip', health spa experience, or more than merely relaxation and un-winding; then:
1. Meditation must be purposive
2. The purpose must be valid and Good
3. The method and style and mechanisms of meditation are likely to be personal - not stereotypical nor standard
In sum, meditation cannot really be taught but must be individually learned by trial and error.
And this is possible because when there is purpose, then each person is able to monitor his own attempts and to evaluate his own success at meditating; especially since there are several other sources to check the results of meditation against for validity - church teachings, scripture, reason, help from trusted spiritual advisers... and so on.
So meditation is an adventure of personal discovery; but like all real adventures it cannot be an aimless, passive wandering; but must have some purpose, direction and point to it.
Meditation will not reveal or teach you the meaning of life - it is one of the means by which you may choose to pursue what you already know to be the meaning of life.