LDPE is short for low-density polyethylene. Its molecular chains carry many long branches, so the chains cannot pack closely together. The result is a softer, more flexible and usually glossier, more transparent film.
LDPE melt tends to form a stable bubble on a blown film line and seals easily. This is why it is often used in gusseted bags, zip-lock bags, packaging films and shrink films. It generally needs a thicker film than HDPE to achieve the same strength. It is also processed blended with LLDPE to increase strength.