Marco Russo and Alberto Ferrari explain the idea of filter context:
This article is part of a series of articles about the basics of DAX. In a previous article, we introduced the first evaluation context in DAX: the row context. If you are not familiar with the row context, we strongly suggest that you start by reading that article first. Here, we build upon your knowledge of the row context to introduce the second evaluation context: the filter context.
Be mindful that the most relevant information you need to master about evaluation contexts is the difference between the row context and the filter context. You cannot appreciate any difference until you know exactly what the two contexts are. This is the reason why approaching the filter context without any existing knowledge about the row context would be only partially useful.
Read on to understand how this all differs from row context.