Malware is simply defined as malicious software. You'd expect bad things to happen to your system environment once malware has entered. Once typical malware enters the system, it does two basic things: installs itself and does its evil work. With the intent of forcing itself to be installed in the system malware does not need to notify the user at all. Instead, it directly makes changes to the system.
One of the changes malware makes in the system is to make itself resident. Malware persistence means that the malware will still be running in background and, as much as possible, all the time. For example, malware gets executed after every boot-up of the system, or malware gets executed at a certain time of the day. The most common way for malware to achieve persistence is to drop a copy of itself in some folder in the system and make an entry in the registry.
The following view of the registry editor shows a registry entry by the GlobeImposter ransomware...