How can I check IMEI on iphone 6 plus and 5s? And please tell me the difference between lock and unlock?

You can check those numbers under the Settings menu of the phone, by clicking on Settings and then General and then About and all that info is there.

An unlocked phone means that you can use it with any carrier you like and a locked phone is locked to a carrier, so if you purchased it from a carrier directly and had it subsidised, then it will be locked to that carrier.