128 bit SSL is also referred to as strong SSL security. The 128 bit tells users that the size of the encryption key used to encrypt the data being passed between a web browser and web server is 128 bits in size (mathematically this would be 2 to the power of 128). Because the size of the 128 bit key is large it is computationally unfeasible to crack and hence is known as strong SSL security.
Most web servers and web browsers support 128 bit SSL. However some versions outside of the US will only support 40 bit SSL and should be upgraded.