"Maven is a Java project management and project comprehension tool" (from the maven homepage).
Maven is a build tool. In many ways it is like the well-known tool ANT, only even more powerful. (Maven uses ANT, so you can still do everything that you can do with ANT). Maven uses a set of project.xml files to work out what to build and in what order. It also manages (and downloads) any third party jars that are needed for a build.
GeoTools is a large and complex project which is split into a number of modules. Without maven it would be very hard for us to manage the build process.
Maven also produces a set of detailed web pages full of information, test reports, javadocs and browsable source code. You can see these online at http://modules.geotools.org
You can find out a lot more about the tool from its home page at http://maven.apache.org