To be timid is to be shy or careful because you feel afraid.
For example, you might feel timid on your first day at a new school, where your teacher and your classmates are strangers to you. Because you feel timid, you stay quiet, and you move slowly or carefully, afraid and unsure.
If you try to pet a timid cat, it will run away, or even hide. It's scared of you, so it's careful not to get too close to you.