It is your 'attention span'. It also has a rapid forgetting rate. We combat this rapid forgetting rate by repetition, as in repeating a phone number, a person's name, a travel direction and so on. The best way to improve it is to learn to 'chunk'- the ability to group long lists of data into smaller chunks. For example it would be difficult to remember 109244153.
Many people confuse long-term memory with events which happened a long time ago.
Although long-term memory holds data that was learned 30 years ago, it also holds material learned 30 minutes ago. Its basic difference from short-term memory is that it is permanent. It holds items as varied as your birthday, what a now-deceased relative looked like, which keys on your key ring fit which locks, how to operate your microwave oven and your husband's new phone number at work. Permanent memory is like the filing cabinets lining the wall. The in-basket has a limited capacity.