As you note in your question, to "convene" is to come or bring together for a meeting. So to "convene a conference" is got get a bunch of people to come together for a conference.
But you're quoting an inapplicable definition of "resolution". The relevant definition is, "A formal statement of a decision or expression of opinion put before or adopted by an assembly". A "resolution" in the relevant sense is like a law passed by a legislature: Some formal organization gets together, takes a vote, and passes a resolution.
(The difference between a "law" and a "resolution" is that a law typically requires people to do or not do something under penalty of fines, imprisonment, execution or some other penalty. A resolution says that the organization itself intends to do something, or calls on others to do something that it has no power to force them to do. Like, the U.S. government could pass a law forbidding U.S. citizens from selling weapons to a hostile nation. But it couldn't pass a law preventing, say, Britain from selling weapons to that nation -- the U.S. Congress has no power to force Britain to do anything. It could pass a resolution stating that it requests Britain not to sell weapons to this country. Likewise, organizations that have no power to pass laws often pass resolutions stating their intent to do this or that. Like pass a resolution saying we will donate $X to this charity.)
So in this case, the United Nations General Assembly passed a resolution calling for this conference to be convened.
