Player Level is basically a measure shows your progress in the game. You can earn XP by doing actions, and once earned XP reaches certain amount, you get your level up. You can check Player Level and required XP to reach next level at the top left-hand corner. SimCash and unlocked objects, locations, and so on are rewards for leveling up.
How to earn XP
Do some actions! By tapping a sim or a object, you will be asked to do an action. You can check what you will spend for it and how many XP you can earn.
Actions take for a while to complete. You can use SimCash to bring it to an end instantly. Note that however SimCash is valuable, so I don't recommend you to waste it.
Once an action is done, a check mark appears on your head. Tap it to complete and receive reward.
After tapping a check mark, reward is given to you. You don't have to tap them to pick up.
How to get your level up faster
Complete Daily Wishes!
Achieving Daily Wishes is the fastest way to get your level up. Three Daily Wishes are assigned each day, and you can earn 35 XP by each one. There is no penalty even if you miss them.