Your question's has multiple answer since they already exists everywhere and easy to get them I am just pointing you to them with links and little description.
What is Maven ?
- Maven is a build tool,it helps to manage dependencies (jars) easily and efficiently.
- Maven does everything what ANT can do,even it has plugin to do some specific ANT activities.
- Very easy to migrate from third-party library.
- Unit testing also bit easy to configure,overall configuration is very simple.
For more information -> http://maven.apache.org/what-is-maven.html
Why Maven ?
https://stackoverflow.com/questions/1077477/why-do-so-few-people-use-maven-are-there-alternative-tools
Why resources directory ?
Before maven exists we used to have resources package within application,which will actually contain some application configuration properties file,Internalization property files , XML files (basically non java files which are required for application run-time ).
One case study i were been using Resource directory
In our company we wanted have different property values for development,staging, pre-production,production environment.So we had four directories and Resources folder saying environment name.Each directory will contain its own property files and properties.During maven build we should mention which profile is this build for ?.Based upon that only specific resource files are loaded in the deployment assembly.
More information about profile in maven -> http://maven.apache.org/guides/introduction/introduction-to-profiles.html
Eclipse error's solving
And your eclipse is having red flags you need to fix the build path issues,if it gets the respective source code.Red flags will be vanished.Since you didn't mention what errors you are seeing i am placing general discussion on build path fixing - > Problem with Java Buildpath in Eclipse