The metaphysical poets were also known for their use of conceits, using an extended metaphor as the basis for the poem in order to make an argument. In "Valediction", Donne uses the image of a compass in order to explain that the souls of himself and his beloved will always remained joined, no matter what corner of the earth they might separately be at.
If they be two, they are two so As stiff
twin compasses are two;
Thy soul, the fix'd foot, makes no show
To move, but doth, if th' other do.
Their souls are the feet of the compass, meeting in the middle and never moving off their spot - "fix'd". This is because the souls are joined by love.
The metaphysical poets wrote poems concerned with metaphysical issues. That is to say, they wrote about things beyond the natural, rational, and/or empirical world, having to do with the ultimate nature of reality. Donne's poem definitely does this. It starts with the address of the soul in the second line; the humans talked about have souls, and talk to them. That in itself would signal metaphysical concerns. The focus on death moves this into the metaphysical, especially since Donne talks about things existing beyond death. Finally, the emphasis on religious faith, especially transcendent faith, makes this metaphysical.