Linux likes to cache every file that it opens. Every time you open a file for reading, Linux will cache it but it will drop those caches if it needs the memory for something more important -- like when a process on the system wants to allocate more memory. These caches in memory simply make Linux faster when the same files are used over and over again. Instead of actually going to disk every time it wants to read the file, it just gets it from memory and memory is a lot faster that disk. That is why your system shows 25493068 used in buff/cache but also shows 25311948 available. Much of that cached data can be freed if the system needs it.
