According to the National Law Review there are several major changes, including the following:
Increasing the number of judges and reducing their retirement age – The number of the judges of the Supreme Court will be increased from the current 81 sitting judges to at least 120. The current retirement age of 72 will be reduced to 65. The right of a judge to continue to be active after the retirement age after providing evidence of good health is limited to judges who receive the consent of Poland’s President to remain active. Because approximately 30 of the current 81 judges are above the age of 65, these two changes will mean that a new majority of judges on the Supreme Court will need to be appointed.
Changes to the method of appointing judges – Judges of the Supreme Court will be appointed by Poland’s President, following their nomination by the National Council of the Judiciary. In separate legislation, the parliament has changed the method of electing members of the National Council of the Judiciary. Before, a majority of the members of this council were judges chosen from assemblies representing various levels of the judiciary. Now, Poland’s Sejm (the lower chamber of the parliament), will have the right to choose 15 members of the council – a majority — from among Polish judges, thereby ending the dominance by members of the judiciary to nominate judges to the Supreme Court.
This means that now not only a majority of the Supreme Court judges will have to be appointed, but also that the Sejm being able to replace the majority of the National Council of the Judiciary means that (indirectly) the PiS can appoint the majority of the Supreme Court with judges that are PiS-friendly.
Note that it is not unusual in Western countries that the legislative can appoint judges (although this has been repeatedly critized), so this alone is not remarkable.
But, as Denis de Bernardy already stated in his answer, judges typically are appointed by lifetime, which makes it virtually impossible for a government to install only judges suitable to them. By contrast, the PiS is now able to install a majority of judges that suits them - and is able to fire them again when deemed necessary. This severely limits the independence of the Supreme Court and thus violates the separation of powers.
Note that it is not one aspect alone that is the problem, but the combination of all the aspects. You may find each of the aspects (legislative nominating the judges, executive being able to fire judges, judges not nominated for lifetime etc.) in some democratic states, but not all at one.