Until now, Mormon missionaries have only allowed to use phone calls or video chats to reach their families on Christmas and Mother's Day. After that, they'd had to rely on email or traditional letters. They'll have an easier time staying in touch from now on, though. The Church now permits missionaries to call, text, message or video chat with their families each week on preparation day (effectively, their break), not just two times a year. They're also encouraged to get in touch on other holidays.
There are restrictions beyond the timing. The Church wants missionaries to initiate any conversations, and asks them to use "good judgment" on the length of calls and chats. They're also told to be respectful of how this affects their missionary companions.
The Church is direct in explaining why it's loosening the rules: the widened communications should help keep families in touch and give a boost to missionaries who are feeling homesick. Indirectly, it's also an acknowledgment that communication technology has changed -- there's a whole generation of missionaries that has grown up with smartphones and may prefer a video call to an email.