MSS is a OSI Layer-4 data size (in bytes) any segment can carry which doesn't include transport layer's headers where as MTU is a OSI Layer-3 IP packet size (in bytes) which can be transmitted through an interface.

for example if we have

TCP SEGMENT for IPv4 will always have MSS as 1460 bytes and IP MTU of interface as 1500 bytes, which explains that data size can not exceed 1460 bytes in a single segment of TCP and 1500 bytes in a single packet which contains that segment. 

TCP wireshark capture is as following: