Destinations represent certain endpoints in your system that live outside of MeV, like the virtual MIDI port of a software synthesizer or a physical MIDI output.

Parts can contain many different events (like notes, pitch bend, controller data, etc), each potentially going to a different destination. This is an uncommon convenience. Most MIDI sequencers require you to organize your events by the port and channel that they will be transmitted on. MeV breaks down this barrier and allows you to organize your events in a way that fits the structure of your music.

When you launch MeV for the first time, you should see the Assembly window, which is the main interface for song arrangement.

The Assembly window is a special variant of the generalized window framework used in MeV to display different types of events in musical time, allowing multiple components, dubbed Strips, to be inserted into the editor window. A similar kind of window is used for editing individual parts.

In most MIDI sequencers, you have a variety of ways you can view a track or sequence of notes - 'piano roll' editor, drum editor, event list, etc. MeV integrates these diverse ideas into a unified approach that utilizes "nested" editing contexts, wherein a high-level editor window can have several smaller tools or components nested inside of it.
Each strip is an object that can represent a different aspect of the musical performance, such as note pitch, note velocity, aftertouch, repeat bars, and any other aspect of the musical performance. Each strip can be vertically scrolled and zoomed independently of all the other strips, but in horizontal scrolling the strips always move in sync. By adding and deleting strips, the user can customize the editing display to see desired parameters and hide unwanted information.

For an extended explanation of MeV's interface read the interface section.