What is SMB?
SMB stands for Server Message Block. It's a network protocol that provides communication between computer and server.
SMB which works on application layer in OSI model, is used to access file share, network, printer, and various connections. Someone can gain admin access by using exploit or malwares via SMB protocol and install trojan, ransomware, etc. on your computer.
Worst-hit country is definitely Russia. Banks, interior and health departments, and state railways are most injured parts. Taiwan and Ukraine follows Russia. It's known that WannaCRY malware used this vulnerability to infect systems.
Vulnerable Operating Systems
This vulnerability is fixed in 2017 Windows update (search:MS17-010: Security update for Windows SMB Server). That protocol comes closed as default anymore in new systems but it might be still open? It'ld be better if you check it.
What You Need To?
Search Windows Features, find and disable 'SMB 1.0/CIFS File Sharing Support' if it's enabled. If you did that, you need to restart your system. Then you're good to go.
To have a secure computer:
Don't use cracked softwares
Use open source alternative softwares
Be a conscious user
Update your system in time