Rising action is the development of drama or conflict within a story. Rising action relates to the hurdles that stand in the way of a character's goals and how he eventually overcomes those goals.
All protagonists have a goal; without a goal, there is no story. The obstacles that stand in the way of each goal, whether it's to win a girl's heart, solve the mysteries of the universe or dethrone a tyrant and place the proper ruler in power, are the rising action of a story. Rising action can be any obstacle, no matter how small or large it is.