Beginning writers (and not-so-beginning writers, for that matter) are often inclined to tell all, to explain everything. For example: "The reader has to know that my main character lived in a yellow house with petunias in the yard until he was seven, and that he had a cat named Mortimer, and that he broke his arm in the sixth grade."
Well, maybe the reader needs to know all that, but probably not. Meeting a character is like meeting a new person in real life. We don't expect everyone we meet to sit down and recount his or her life story to date. If there's something really big in this person's past, especially something that relates to us, we need to know and we tend to find out fairly quickly. But mostly, we make up our minds about people by watching them in the present: observing what they do, how they treat us and others. We can get to know characters the same way. Bits of the past will come to light, especially if we get very close to a character, but they will arise naturally.
Readers also engage with a text more when they get to put some of the pieces together themselves. Part of the delight in reading is filling in the blanks, constructing an inner world. In fact, there is a whole group of readers whose enthusiasm leads them to continue filling in blanks on a much grander scale: writers of fanfiction.