Mary Sue, by definition from Wikipedia,
"A Mary Sue (sometimes just Sue), in literary criticism and particularly in fanfiction is a fictional character with overly idealized and hackneyed mannerisms, lacking noteworth flaws, and primarily functioning as a wish-fulfilm
ent fantasy for the author or reader. It is generally accepted as a character whose positive aspects overwhelm their other traits until they become one-dimensional. While the label "Mary Sue" itself originates from a parody of this type of character, most characters labelled "Mary Sues" by readers are not intended by authors as such. Male Mary Sues are often dubbed "Gary Stu", "Larry Stu", "Marty Stu", or similar names."
To avoid your character being a Mary Sue, there are online tests to help determine how well rounded your character is. Remember, everyone has flaws, so don't be afraid to allow your character have real flaws, not something silly like being clumsy. We are looking for a balance between good and bad traits, so try and think your character through thoroughly.
Also, if your character has relationships with any canon characters, think it through thoroughly. Remember that this is your OC, so please don't create your entire character based off a canon character. We want you to be original, so try your best.