FPS stands for Feature Pack Stack
Example: S/4HANA, on premise edition (1511 FPS 2)
For the product SAP S/4HANA on premise edition, SAP is shipping 1 new key release per year, and 3 successive Feature Pack Stacks (FPS) on a quarterly basis. Then comes the next key release, followed by 3 Feature Pack Stacks, etc.
Technically speaking, an FPS is similar to an SPS(Support Pak Stack), but it may include non-disruptive, non-mandatory features.
Here concretely for the current on-premise product:
SAP S/4HANA, on premise edition 1511 (shipped in November 2015)
Then SAP S/4HANA, on premise edition 1511 FPS01 (shipped in February 2016)
Then SAP S/4HANA, on premise edition 1511 FPS02 (planned in Q2 2016)
Then SAP S/4HANA, on premise edition 1511 FPS03 (planned in Q3 2016)