What does WCDMA 2100/1900/900 on a nokia e5 mean?

Short answer
this tells you that the phone is what is know as as tri band phone and will work on any of the 3 stated UMTS networks frequencies.
check country of travel to ensure compatabilty.
long tech answer.
the term Wideband Code Division Multiple Access means that you phone is code divisional and will only react to a call that is made that is for your phone, even if more calls are being received on the same frequency the other are ignoted and you have full security of your call. you phone can decipher which is the call for it and accept the call this is used to save on network frequencies. under the
wireless Universal Mobile Telecommunications System networks.

2100 is the mhz range 1920-2170 mhz as used in Europe, Asia, Africa, Israel, Oceania, Brazil, India
1900 is the mhz range 1850-1990 mhz as used in Americas, Chile, Venezuela, Colombia
the 900 is the mhz range 880-960 mhz as used in Asia the carabien and europe.

so as long as you have a roaming agreeming with your local provider this phone will work in most places in the world.