These applications replace the standard dialer and aggregate your contact information from both local and public Internet sources to add names and photos to both incoming calls and to your call history - even for numbers which are not in your local address book. It is really useful functionality.
I have been using a similar application called CallApp on my Samsung Galaxy S6 for the last few months, but will have to give Truecaller a whirl now, given their support for Tizen!
There is the beginnings of a Tizen FAQ on their website. They also have a Blog and a Tech Blog. It appears that the company has a history of supporting alternative mobile OSes, with support for Symbian, Blackberry, Blackberry10 and even Series 40! Good for them.