While connecting to your server through SSH can be very secure, the SSH daemon itself is a service that must be exposed to the Internet to function properly. This comes with some inherent risk and offers a vector of attack for would-be assailants.
Any service that is exposed to the network is a potential target in this way. If you pay attention to application logs for these services, you will often see repeated, systematic login attempts that represent brute-force attacks by users and bots alike.
A service called Fail2ban can mitigate this problem by creating rules that automatically alter your iptables firewall configuration based on a predefined number of unsuccessful login attempts. This will allow your server to respond to illegitimate access attempts without intervention from you.
checkout https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-centos-7 for a detailed fail2ban installation