Hemoglobin. Is what carries oxygen to the entire body. Without enough every single cell is being shorted. Some are more sensitive than others - brain, heart, eyes.

Low hemoglobin is very dangerous.

Hemoglobin carries oxygen to your body and when its low it makes the heart work harder. That can cause irregular heart beat which can further lead to things like heart failure.
ELI5: Not enough oxygen kills you.

On top of hemoglobin carrying oxygen, it can also be a sign of an underlying condition. For example I was severely anemic and it was Leukemia.

Ok. I have another question. They did a blood test with 0.5 ml of the blood within 5 minutes of us entering the emergency ward and that’s where they found low hbg. Do they check for leukaemia in this test. Sorry if this is too particular

Hemoglobin carries oxygen to every cell in your body. Low hemoglobin means less oxygen is getting to your cells. Different cells can go do long without the optimal level of oxygen they need, and so, those parts of your body get damaged. The amount of damage done to different parts of the body determines the effect on your overall health. Since certain parts of your body have really important functions and a high demand of oxygen (your brain, your heart) this could have especially bad long term effects.

There are definitely blood tests they can do to detect markers for certain cancers but it’s impossible to tell if they did the test for leukemia if we don’t know what the test is called… what is the test called that they did?

Cancer screenings usually take longer. Depending on the type of cancer some blood tests may pick up on it. Some require specific tests. Hard to know without knowing exact testing that was done. It could be an iron deficiency or recent infection or something simple all the way up to bone cancer or leukemia etc. any diagnosis should be from a doctor directly involved in their care with all the info.

Haemoglobin is an important chemical in your red-blood-cells. It is the chemical they use to carry and deliver oxygen in your blood.

If you are low on haemoglobin, then I think you'd be low on oxygen, no matter how much you breathe.

The quick test will be a point of care test (like a glucose meter but for hb) - they'll then collect more blood to check much more detailed tests - these will give good indications of why the hb is so low - blood count is pretty helpful for leukaemia as you'd find way too many of one type of blood cell. Most common is low iron, also possible low b12. Then there's other things but basically boils down to not making enough blood, or losing too much.

This is an excellent explanation. To provide some context, though, a hemoglobin of 6.6 is not all that low. It’s certainly low enough that the person should get transfused a unit of blood (usually indicated if the value is below 7). However, the real issue is figuring WHY it got low in the first place. Is the patient’s own immune system attacking their blood (autoimmune)? Is there a bleed somewhere (like the gastrointestinal tract)? Is there cancer in the bone marrow leaving (less likely if only the red cells are low but not the other blood counts)? Is there an iron deficiency for some reason? Figuring out the cause of the anemia will be important to knowing how to treat and the actual prognosis.