A postcard poem is quickly written, short and sweet. Think "haiku" without the syllabic considerations. Or, if you must, think about it as an old-school tweet. You simply write a poem on the back of a postcard, any postcard, any subject. You can write about the picture on the card, or what is going on around you at the moment. The idea is for the poem to have a feeling of immediacy, of present-tense.
Once you have gotten down your poem, mail it, or leave it in a strategic place for someone to find. Put it in a bottle and throw it in the nearest body of water. Leave it under a windshield wiper. Tie it to a balloon. Set it free.