Backseat gaming is when someone watching another person play a video game decides, hey I know better than them I’ll help them out. However, when put into practice it becomes less help and more half-finished directions yelled in your ear.
You've just settled in for a nice 4-hour Halo 3 marathon you've got a packet of Doritos's sitting next to you, your all set. You get into your first match after trash talking some 12 year olds in the lobby, your ready to own. All of a sudden, your sibling/friend walks in, and it begins "I love this map; oh I know this killer sniper spot you should go to it." So you politely ignore them hoping they might just go away, but it has just begun. "Yep turn right, right, NO YOUR OTHER RIGHT! Quick, quick, quick, shoot him, shoot, shoot, aw I would have killed that guy." Seriously if you ever find yourself doing this, just follow this easy three step process, Realize you're doing something wrong, Stop doing said thing, and Apologize and leave.
This is always one of those "now the tables have turned" moments, so much so that I'm not even sure if people actually backseat game or it's just a series of tit for tat retaliations, either way it's intenesely fun and you should try it. And don't let inexperience or lack of ever playing the game stop you from helping, no, that's what normal people would think.