Saying "when it comes to ___" is a way to limit the topic of your sentence. For example:
James is a genius when it comes to computers.
This means that James is very smart, but specifically on the topic of computers. On other topics, James might be smart or maybe not.
"When it comes to ___" can go at the beginning of a sentence or at the end:
When it comes to eating, nobody beats Mako!
I'm pretty much worthless when it comes to computers.