A good question. There is an entire book that answers this question:“Fire Within” by Thomas Dubay, a Catholic priest. Some excerpts from it:
(Page 57) Christic contemplation is nothing less than a deep love communion with the triune God. By depth here we mean a knowing loving that we cannot produce but only receive. It is not merely a mentally expressed 'I love You". It is a wordless awareness and love that we of ourselves cannot initiate or prolong.
(Page 7) There is is another segment of individuals who have dabbled in Christian and oriental mysticisms and consider that they are more or less indistinguishable. An expert will not, of course, make this astonishing mistake, but others notice similarities (for example, an asceticism, an imageless awareness, a reaching out for the transcendent), while they fail to recognize vast gulfs between the two. Buddhist “contemplation” is impersonal, not a love matter at all, whereas that represented by (Christian mystics) is preeminently a profound personal love union with God.
Because we’re on the apologetics forum, I’ll note that many Protestants have criticized Catholicism by saying that Christianity is “a relationship, not a religion”. I will observe that contemplative prayer as experienced by Catholics perfectly fulfills this “relationship”.