Hi Leriel,
Daylight saving is the practice of putting the clocks forward an hour, usually around March and then putting them back an hour, usually around October.
It is not a universal practice - but was introduced by some countries during the First World War and then repeated during the Second World War and maintained by them ever since. By getting people out of bed an hour earlier it is supposed to provide longer daylight working hours.
For Astrologers this leads to the problem where recorded birth times are an hour out from Solar time. To avoid complications of longitude, suppose someone was born in London on July 1st at 12:00 by the clock. During July the UK is on British Summer Time (BST) which is one hour ahead of Greenwich Mean Time (GMT)
GMT is actually the local Solar Time in London - so true Noon would not actually occur till 13:00 BST. In order to establish the positions of the planets at the time of birth (and indeed the MC and the Ascendant) we need to make use of an ephemeris - and these usually give the planetary position at either midnight or noon GMT. So if we used an ephemeris for noon GMT and used the recorded birth time of noon BST unadjusted, the natal chart would be an hour out.
This is a common error - I've had it from professional organisations. If you are drawing up a chart for yourself or someone else you may find they know their birth time but they won't tell you as a matter of course whether daylight saving was being used - its your job as the Astrologer to check.
A number of programs are sophisticated enough to know the changes but many require you to tell them what time system is in operation. I'm not sure what the situation is in Poland but if you do adjust your clocks twice a year then you need to make allowance for daylight savings. Even if you do not you may need to do a chart for someone born abroad, where daylight saving is used.