A "smurf account" is a secondary account where you are at lower ELO than your original account. This means someone that already is level 30, and most times already ranked, is playing in a low level account, or someone in high ranked divisions is playing in lower ones. It's often used when someone that's really good is stomping a game.
While it's almost always the case, the smurf doesn't have to be lower rated. Last season I got to high diamond on my smurf while my main was still plat 5. Main account = the account you always play on, use for normals etc., smurfs = any additional accounts you use, usually just for ranked.
To simplify a bit more from Ekki's comment: it's basically anyone who has more than one account, so the lower leveled or lower ranked one would be considered a smurf account, since the player is better than their level/rank would imply.
For example, I am level 30 and Gold ranked. If I made a new account, and was at level 15, that would be my smurf and theoretically I'd be better than anyone else I would play against.
It also applies also for ranked games.. For example If I continued to play on that account and reached level 30 and started playing ranked games at Bronze rank, I would still be smurfing since my main account (lvl 30/Gold) is still higher than my smurf account (lvl30/Bronze).
For example, I am level 30 and Gold ranked. If I made a new account, and was at level 15, that would be my smurf and theoretically I'd be better than anyone else I would play against.
It also applies also for ranked games.. For example If I continued to play on that account and reached level 30 and started playing ranked games at Bronze rank, I would still be smurfing since my main account (lvl 30/Gold) is still higher than my smurf account (lvl30/Bronze).