2000 Chevy Blazer PO300

I took my car into the shop. They tested it and it came up with PO336. Replaced the Crank sensor. Drives great now. Or at least alot better then it did. My service engine soon light came on periodically over the last 2 1/2 years. Sometimes would go off for months at a time. They fixed the po336 problem. After I got my car back my service engine light was coming on again. PO300. I took it back to them. They said they checked all the spark plugs and spark plug wires and they are fine. The car is running great and that the Servicce engine light, I should ignore. What advice do you have for me? Any ideas? I going on a road trip at the end of June and I'd like to feel safe taking my SUV. I don't know much about cars. So it would be appreciated if you could talk to me as if I am 5. Thank you.