C IP means C class IP. It is the 3rd number in an IP address (which is basically the address of your computer or server on the internet). When 2 websites have the same C IP suggests that they are on the same host.
Why does this matter?
Here are 2 examples of how it can matter:
First, if you own both websites and they are linking to each other, then Google sees this as an indication that they might be owned by the same person. This might lead to less linkjuice or even a penalty.
Second, you might notice that a lot of links are coming to your site from websites that have the same C IP. This could indicate a coordinated effort by an individual to send you a lot of links (often quite spammy ones). In that case, you should carefully analyze those links and consider disavowing them to combat negative SEO.