Question about super large number units in hubbles constant

[zeromodz]

This is hubbles constant from my source
74.2 ± 3.6 (km/s)/Mpc


What does the ± mean?

 

[Integral]

That means that the value of the Hubble constant lies somewhere in the range of 74.2 -3.6 and 74.2 + 3.6. Every measured quantity has errors due to method and instrumentation called the error bounds.

For example, when using a ruler we usually assume an error equal to 1/2 the smallest division mark. Let's say you use a standard meter stick for a measurement of 10cm. Your error bound would be specified by 10 +/- .05 cm