What does this equation mean?

Data Center TCP refined the marking policy for the latencies and buffer sizes actually found inside a datacenter. Its authors describe DCTCP as a scheme that “leverages Explicit Congestion Notification (ECN) in the network to provide multi-bit feedback to the end hosts” rather than the single congestion bit ordinary TCP effectively gets from a drop [ 14 ] . The practical mechanism is a threshold rule applied at each switch queue: writing Q for the queue’s occupancy sampled at a packet’s arrival and K for a fixed marking threshold, the switch’s marking decision m is $m(Q) = \begin{cases} 1 & Q > K \\ 0 & Q \le K \end{cases}$. so the sender receives, over many packets, a running estimate of what fraction of its recent traffic found the queue already over threshold — a proportional read on the severity of congestion rather than a single bit saying only that congestion happened somewhere.