what do they mean by 3.3V CMOS
I am using some CP2102 USB<->serial devices. These run internally at 3.3 Volts and only output signals at 0V and +3.3V not the voltages specified in the RS232 spec. So far as "CMOS" goes, I expect that is meant to imply they won't drive a standard TTL load, just a lower powered CMOS input.
The RS232 serial standard is inverted and has voltages in the range of +/- 25V. The '0' is e.g. +10V and the '1' might be -10V.
The 3V3 CMOS or TTL serial use non-inverted signalling, i.e. a '0' is 0V and a '1' is 3.3V (in the case of 3.3V CMOS).