Apple's interest in Imagination comes down to their PowerVR graphics processors, which Apple continues to utilize within their in-house ARM chips. If buying out Imagination, Apple could now have their graphics tech in-house as well. This rumor isn't too outlandish considering Apple already owns a minority stake in Imagination (just under 10%) and has made similar acquisitions in the past for their SoCs that power iPhones and iPads.
It will be interesting to see if the rumor pans out and what Apple ends up doing with the company considering outside of the PowerVR series, which Imagination continues to license to other non-Apple customers, they own MIPS and other product/IP line-ups. This acquisition would likely be sad news for their Linux driver efforts they've been ramping up for with a supposed open-source driver potentially coming in the future. But in time anyhow the future PowerVR graphics tech would likely be proprietary to Apple anyways and fade into irrelevance for Linux users.
Some more commentary can be found on Bloomberg about these rumors that are exciting investors this morning.