Little annoys me more in a book than authors who tell me something they could easily show me. Or possibly worse is telling me alongside showing me.
This is a silly example, but it's the book I've read most recently, and it'll work.
In - er - Curious George Bakes a Cake the author says:
"What was that strange looking gadget? George was curious."
We don't need to be told that George was curious because the author already showed us when George wondered what the strange looking gadget was. And which is more interesting to read? George's actual thoughts about the gadget, or the statement that he's curious? (Although if you've read the book a thousand times to your toddler, none of it feels very interesting anymore.)
Don't tell me your character is angry - show me.
Don't tell me your character is jealous - show me.
Telling - When she spotted Jenny flirting with her boyfriend, Katie felt angry.
Showing - How dare Jenny flirt with her boyfriend! Hadn't they vowed back in seventh grade to never let a boy come between them?
Telling - Katie often felt jealous of Jenny.
Showing - As Katie observed the bounce of Jenny's glossy, blond ringlets, her chest tightened with an all-too-familiar feeling . She smoothed her frizzy red locks and reminded herself that it was a person's heart that mattered, not their hair.
And this goes for deeper issues too. In newbie manuscripts (and, sadly, some published books) I often read paragraphs like this:
Katie and Jenny were really good friends, even though Jenny drove Katie crazy sometimes. Even though Jenny had a bad habit of ditching Katie whenever cute guys were around, Katie would always be there for her. No one understood why Katie felt this way, but they didn't know what kind of childhood Jenny had.
If you find chunks like this in your manuscript - cut them!
These are just details of one of your plot lines. The reader doesn't need this spelled out for them. Over the course of the story, you can show them that Jenny drives Katie crazy. You can have scenes where Jenny ditches Katie for cute guys. You can then show a situation where Katie's been treated horribly, but comes to Jenny's rescue anyway. You can have friends say to Katie, "Why do you put up with Jenny?"
Trust your reader to be smart enough to not have everything laid out for them in black and white.