My grandpa is going in for a test tomorrow because they think there is a blood clot on his lung. What is this? What are the effects?
Please help me, I am very worried

Blood clot - "A pulmonary embolism (PULL-mun-ary EM-bo-lizm) is a sudden blockage in a lung artery, usually due to a blood clot that traveled to the lung from the leg. A clot that forms in one part of the body and travels in the bloodstream to another part of the body is called an embolus.
Pulmonary embolism is a serious condition that can cause:
Permanent damage to part of your lung from lack of blood flow to lung tissue
Low oxygen levels in your blood
Damage to other organs in your body from not getting enough oxygen
If a clot is large, or if there are many clots, pulmonary embolism can cause death.
In most cases, pulmonary embolism is a complication of a condition called deep vein thrombosis (DVT). In DVT, blood clots form in the deep veins of the body—most often in the legs. These clots can break free, travel to the lung, and block an artery." Definition taken from www.nhlbi.nih.gov

So that's the scientific bit out the way. Now for my personal experience:

3 summers ago I had multiple blood clots on both my lungs. There was no apparent reason for them, and my symptoms were more like asthma - difficulty breathing, wheezing and then I nearly passed out one morning. The doctor fortunately recognised that it was not asthma and referred me immediately to hospital, where I was put on oxygen and sent for a VQ scan. This showed clearly that I had many clots on both lungs. I was treated with Heparin injections and bed rest for a week, then when I was discharged I injected myself with Heparin for a further 10 days (I chose to do it myself, I could have gone back to hospital each day) then I spent 6 months taking Warfarin. (Heparin and Warfarin are blood thinners)
After the 6 months, I had pretty much completely recovered, though I still get breathless sometimes. I now take garlic and ginkgo biloba supplements every day and eat lots of garlic, as these are natural blood thinners.
I think the fact that your grandad is being sent for a test is a good thing. It means someone has diagnosed the problem (probably), it will be identified quickly in hospital and treatment can start fast. I understand that the many people die from these clots because they go undiagnosed because the symptoms are vague and unspecific. I would be heartened by the fact that he is about to get medical help.
Good luck.