This is the single most important plugin to install, activate and configure - if you have a WordPress site.
It does a lot of things, the most important of which are:
Limits the number of failed logins (default 20 - but you set) into you member's account and locks out (default 4-hours but you set) for that ip address. Hackers spend a lot of time trying to vandalize an account. Be sure to White list yourvIP address.
Is an anti-virus for your account. Keeps you (or a hacker) from uploading a file with a virus into your Word Press account.
Malware scanner/repair
If you have a WordPress running in your account - it's the firewall security plugin I suggest. The full details are available in your add plugins area. They push you to buy their Premium version for more features, but I've never felt the to.