A lot can go on in the mid lane. It is dangerous because it is in the middle, and attack can come from either direction. But you may also leave and attack at either direction. Besides jungler, a mid laner must also learn to gank other lanes in case jungle is too busy. Mid laners tend to use champions that do big bursts of damage and can move is and out of combat quickly. This often comes in assassins ans mages.

During the first 30 second of the game, you will be waiting for minions and monsters to spawn. The pictures shown are where a mid laner would usually wait for minions and get vision on the enemy.

CS means combat score, and you gain cs by killing minions or monsters. CS only counts by killing, not damaging, because an ally minion or champion can steal the kill. CS will grant you gold. You can think of CS as last-hitting a minion for the gold. Otherwise it will be wasted and no one, or someone else, gets the gold. You want that gold because it stacks up.