2017 iMac with lines in display

Very difficult to say. You should take the computer to your local Apple Authorized Service Provider to be professionally diagnosed. If it is hardware, then the you would need to replace the main logic board which essentially means to replace the whole computer.

You can try some software basics such as resetting the NVRAM and restarting in Safe Mode though. If you do not know how to do these reset please click:

NVRAM Reset & Safe Mode Reset