Definition of Inactive memory:
The total amount of buffer or page cache memory that are free and available. This is memory that has not been recently utilized by any application for some time and can be reclaimed for other purposes by the paging algorithm.
The Inactive pages are the first candidate for eviction should the system come under memory pressure. High values are not harmful and it is a general behavior of Linux to keep as much cached as opposed to free as possible (it's for better performance in the case of frequently-used items).