Brute Force attacks
Brute force attack means to use of a program to attempt to guess passwords and log in again and again until it succeeds. They usually try with username admin and use some kind of dictionary to guess the password. Those who do this have bad intentions. Either they just want to destroy your site or more likely put any malicious code in your system.
The first thing you should do to protect themselves is to create a new administrator user named something else than admin and use secure passwords. Passwords should be long and random, therefore not notable words.
A plugin I like is WP ReCaptcha Integration. Unfortunately, it has not been updated for a full year so there is a risk that it ceases to function soon. When that happens try to find a similar plugin. The plugin WP ReCaptcha Integration adds a small box for the user to click.
This integrates Google's free service reCAPTCHA which just by studying the user's mouse movements can determine if it is a human or robot.
For increased protection, you can install a security software such as Wordfence. This blocks users after a specified number of login attempts. How many you set yourself under Options.
A disadvantage of Wordfence is that it uses quite a lot of resources which will slow down your site to load into your visitors ' computers. Another disadvantage is that the application uses IP numbers to block a suspected user but it is not difficult for an attacker to use several different IP numbers. They may very well send multiple login attempts and use new IP number every time.
So do not rely only on a security program, but be sure to follow the advice above with good passwords and your own administrator user.
Security software such as Wordfence has more functions than just protects against brute force attacks, so they are good to have anyway. I will return in later posts what Wordfence is doing and how it is used.