This Maximum Segment Size (MSS) announcement (often mistakenly called a negotiation) is sent from the data receiver to the data sender and says "I can accept TCP segments up to size X". The size (X) may be larger or smaller than the default.  
The MSS can be used completely independently in each direction of data flow.  The result may be quite different maximum
sizes in the two directions.

Below you can see an example where 192.168.1.100 is saying he can accept a MSS of 1280 for all future TCP segments. And 10.1.1.100 can only accept MSS`s of 1460 for all future TCP segments.   

10.1.1.100  192.1.1.100   TCP   39884 > 22 [SYN] Seq=0 Len=0 MSS=1460 TSV=556758839
192.1.1.100 10.1.1.100    TCP   22 > 39884 [SYN, ACK] Seq=0 Ack=1 Win=16484 Len=0 MSS=1280 WS=0  
10.1.1.100  192.1.1.100   TCP   39884 > 22 [ACK] Seq=1 Ack=1 Win=5888 Len=0