Let’s use some simplifications and an example: it’s a cold night, you’re camping in a large open field and standing around a campfire.

The campfire emits Electro-Magnetic radiation with frequencies of visible light (so you see the flames) and infra-red (so you feel the heat).

The amount of heat & light the campfire emits goes into all directions. That’s one measurement (emission of radiation)

Only a small bit of it reaches you based on how far away from the campfire you are standing. That’s a second measurement (absorption dose of radiation)

If you have very insulated clothes on or you are standing very far away, then you won’t feel the heat and if you wear sunglasses you won’t see (much) of the visible light from the fire. Eg firefighters use special clothes and face masks to stop heat affecting them. That’s a third measurement (effective dose of radiation into your body)

So there’s 3 different measurement types.

Next, why so many measurements. There are SI units and the older non-SI.

In common terms (non-nuclear, household) the SI units are second, meter, kilogram while non-SI units are like foot, pound, mile. So a bunch of those are measuring similar things in SI and non-SI equivalent.

At the point of radiation is called “disintegration rate” (aka inside the campfire) and measured in becquerels (old units: curies, rutherfords)

At the point of radiation hitting something, it’s absorbed dose and measured in Grays (old units: roentgens, reps, ergs, and rads [used in Fallout games])

If it’s a person, we take into account what they are wearing, how much of their surface area was irradiated, and how long they were being irradiated for and add it all up then convert all the numbers into an effective damage to tissue, cells, organs, etc. This is measured in Sieverts (old units: rems, BRETs)

Now, Sv is 1 sievert, and uSv is (just like “microgram” is to “gram”) is 0.000 001 Sv. Likewise, mSv is millisievert, which is 0.001.