To be reticent is to be reluctant to speak. It comes from the Latin word for being silent. It can refer to someone who is taciturn - that is, simply doesn't talk much. Or it can refer to someone who is reluctant to speak about something specific. For example, a politician who is being cagey about his plans to run for President might be reticent on that one subject but loquacious (very, very talkative) on all others. Many people misuse reticent when they mean reluctant. They might say, "He was reticent to act." That should be, "He was reluctant to act." Reticent can only refer to speech, not to action.