If something is convenient, it adds to your comfort, causes little trouble, or is easy to use, do, or reach. If you live close to your job, that's convenient––it's very easy to get there and get home.
This adjective descends from Middle English, from Latin conveniēns, from convenīre "to be suitable, fit." The basic meaning of Latin convenīre is "to agree, meet, come together," and this is the source of English convene. The opposite of convenient is inconvenient, meaning making trouble or annoyance for you. The corresponding nouns are convenience and inconvenience.