What is CelebuSerum and how do you get it?

It is the perk right above Noticeable you have to select upward in a branch if you want to get the one above it.

CelebuSerum is a perk for Rising Stars. To get the perk, you must first have Noticeable perk. The icon for that perk are potions with stars and is on the second row.

One of the serums helps the Sim to focus. My Sim has only received about three so far. I don't have him use them.

They're basically the same as the Emotional/Needs potions you can buy in the Aspiration Rewards Shop thingy.

But with two side effects:
1) Your skills level up faster while the effect lasts (4 hours)
2) You have a chance of "withdrawal" when the effect expires, giving a +2 Uncomfortable moodlet for a few hours that makes you learn slower until it goes away (8 hours, I think). Withdrawal doesn't always happen, but can also be ended early by drinking more Celebuserum
You can't actually become dependent on it, and the +2 Uncomfortable moodlet is the only bad side effect

Each celebrity in the house who has that perk will receive a random potion in the mail every day.

Potions come in these varieties:
Confident (+100 Confident)
Flirty (+100 Flirty)
Happy (+100 Happy)
Focused (+100 Focused)
Inspired (+100 Inspired)
Energy (+100 Energized)
Sleep (fills Energy need, +5 Happy)
Fun (fills Fun need, +5 Happy)
Hygienic (fills Hygiene need, +5 Happy)