The lines in question are thickness lines.
Basically they indicate how warm the atmosphere is, as warmer air expands, so the actuall distance between say the 1000mb and 500mb pressure levels will be greater for warm air, then cooler. As a very rough and ready rule of thumb, the one we look for in winter is the 528 line, as precipitation falling in areas north of this will in most cases be snow. In the UK 546, is usual for most of the year, and in summer we occasionaly get 564 or higher during warm spells.
They are a good indicator of air masses. For example, if you see the 564 line pushing up towards say the UK, it shows an area of warmer air approaching the UK. On a similar note, if you see the 528 or even the 510 line coming down from the north, expect a blast of cold(er) air.