Run a Windows program and it’ll often try to access functions or data in a Dynamic Link Library (a DLL file). If the program has provided a copy of the DLL then Windows will load that, otherwise it’ll search various folders on your system to find a copy.
Sounds great, in theory, but there can be problems. If you have multiple copies of a DLL scattered around your system, applications won’t always get the versions they expect. And if malware copies a DLL to a preferential point in the Windows search order – so-called “DLL hijacking” – then you could be running malicious code whenever it’s requested.