Significant weather charts (SIGWX) are generally only used for high altitude flying in jet aircraft. They use a large number of symbols, some of which are outlined below.
Although they can show some interesting aspects of the weather, SIGWX charts are of little practical use in light training aircraft and I’ve never seen a PPL question that required knowledge of them.
If you’re interested, the UK Met Office website has some information on SIGWX charts here: MetOffice WAFC & SIGWX
Here are a few of the symbols on a significant weather chart: