So basically this means using stuff that are outdated and have vulnerablities out for them already like Eternal Blue is an example of this basically.Burp Suite has tools to check for out of date applications for us and show us what things are out of date and have vulnerabilites for us.You can also look at Wapalayzer and see what version of some library its using and then we basically can enummerate and try finidng explits for that version.

To prevent this just patch everything basically.