My shower grout is cracked. How can I fix it?
I used caulk to fix issues like this. You can order it in different colors to fit your grout (amazon link). I also recommend getting a caulk gun. I didn't get one the first time and it was a pain. It is doable without though but caulk guns are cheap and actually pretty useful in other situations as well with different types of caulk. Dry time  is pretty long though. It was annoying to keep the family out of the shower.

Grout versus caulk: I can't really speak to that. I assume caulking is easier but less durable. Maybe someone else can comment on that.