What you are asking is mainly in reference to the GD&T feature control frames on the drawings. Your drawing looks like it's metric, so it may refer to an ISO standard.
If your drawing references a specification, you should research that as this answer would essentially need to teach you GD&T and how to interpret it. Maybe start here:
But there are several resources online.
That being said:
- Machined Surface Finish requirement(material removal required) as indicated by the integrated horizontal line below the value.
- It looks like a datum callout
- Concentricity or Coaxiality(depending on spec and interpretation)(not commonly specified)
- Circular Runout(typically used over concentricity)
What GD&T does is to clearly define what features are important and how they are to be inspected. They are used to control position and/or form of the part. In your case, that's a rotating shaft so typically the cylinders need to be concentric so they are balanced and aligned for assembly.