SE is the written short form of southeast or southeastern: the direction between south and east, or the part of a place that lies that way. On a map you might read SE England; a wind can be a SE wind; some London postcodes start with SE because that corner of the city is the south-east. You almost always meet it on the page, not in careful speech, where people say south-east. Though actually the same two letters are borrowed for other jobs too — a company name, a file label — so look at the setting. If it is a compass, a region, a weather report, or an address, it is this meaning.