"Metaphysical conceits" aren't too strictly defined, but the general idea is that the poet makes use of a clever and unusual extended metaphor throughout much or all of a poem. An extended metaphor, by the way, is just a regular metaphor (directly comparing two things that aren't immediately related) that carries on through more than one sentence. So, in Holy Sonnet 14, the idea of the speaker as a city barricaded against God's advances is a metaphysical conceit. Check out Donne's poem "The Flea" for an even better example.
As for the conflict between sacred and profane love, check out "In A Nutshell" for more on Donne's history with religion. The basic idea, though, is that Donne is really into physical, earthly love, but also really into God and holiness. As you can imagine, these often run into conflict, and Donne likes to write poems that play with this tension.
Let's zoom out a bit. How do the metaphors and the issue of loving God work together? Well, check it out: the metaphors are somewhat strange, even though they're supposed to make the speaker's relationship with God easier to understand by comparing it to other things we know and recognize (war, sex, and an engagement). But, the problem is that the actual action he wants God to take is no clearer to us at the end of the poem than at the beginning. Does he actually want God to “ravish” him? Probably not, right? So, what does he want? The metaphors, instead of making it easier to understand what's going on, just make figuring out what he really wants much more confusing.
And, why make it so confusing? That's where the issue of loving God comes up. The huge problem he must deal with is that he's trying to define a sacred, spiritual relationship, but the only tools at his disposal are the language we use and the lives we lead here in the non-sacred world. The Bible makes a big point of this: the language God uses is not the language we can use, so the kinds of comparisons Donne can make are inherently limited. Our words and metaphors just can't describe what happens when you get close to God. Donne writes about something he really can't express, and that struggle is a big calling card for all of his poetry.