MPPS stands for million packets per second and Cisco prefers to refer throughput in MPPS. For a layer-3 switch an MPPS value is shared one. For some of the higher-end Cisco routers the routing is "distributed" between multiple line-cards, in which case the PPS numbers are based on the number of line cards, bit for non-distributed architectures (Catalyst switches) the numbers are based on the routing engine, so it is the maximum number of Packets per Second that the box can route.
For example, 2960-48PST-S is 13.3 Mpps.
The figure MPPS expresses the maximum number of frames per second that can be processed by the device. It is not dependent on frame size but clearly small frames require higher packet rates.
To give you an idea of what this number says:
smallest frames in Ethernet are 64 bytes in size, taking in account the preamble (8 bytes) and the minimum inter-frame gap (the last two counts roughly for 20.2 bytes) to fill a GE port in one direction you need 1484560 frame per second.
10^9 / [(64+20.2)*8] where 8 is bits/byte.
So a number of 13.3 MPPS is equivalent to [((13.3 M * (64+20.2) * 8)) / 10^9 = 8.95 / 2=4.47] 4.47 GE ports filled with smallest frames bidirectional.
On the other hand frames of max size 1518 bytes require 81264 fps to fill a GE port in one direction.
So this number expresses the forwarding capability of the device.
A non blocking device with 48 GE ports would require 2 * 1484560 * 48 as MPPS or higher.
Therefore the performance of a device will be determined by combination of number of packets per sec and the size of the packet.