Pikuach nefesh is a really important and special Jewish rule that says that saving a life is the most important thing to do, even if it breaks other rules. Pikuach nefesh means that if someone is really sick or injured, or if someone is in danger, it's more important to do whatever it takes to help them than to follow any other rules.
For example, if someone gets hurt on a Saturday (which is a special day in Jewish tradition), Pikuach nefesh says that it's more important to call an ambulance and get them to a hospital right away than it is to wait until Saturday is over before getting help. Or, if a doctor needs to do something that might not be allowed on a normal day because it's against the Jewish rules (like turning on a machine or using electricity), they might still do it for Pikuach nefesh.
The really cool thing about Pikuach nefesh is that it tells us that saving lives is more important than anything else, and that even rules that might be important in other situations aren't as important when someone needs help. So, being kind and helping others is really important, especially if they're in danger or need our help to stay safe and healthy.