What do (2S,3S) and similar notations mean?
When naming a organic compound, S/R-notation comes if there is a chiral atom present. The S/R-notation always accompanies a numerical such as (2S,3R)- where the numerical refers to the chiral carbon position in the molecule according to the IUPAC numbering while R and S representate the orientation of the chiral atom according to the Cahn–Ingold–Prelog priority rules. For example, let's look at glyceraldehyde molecules where all this stereochemistry started from:

The IUPAC name of glyceraldehyde is 2,3-dihydroxypropanal. However, it has a one chiral carbon at $\ce{C}$2 and hence has two stereoisomers. The priority order of groups attached to chiral carbon is $\ce{-OH \gt -CHO \gt -CH2OH \gt -H}$. Hence you can look at the $1 \rightarrow 2 \rightarrow 3$ priority group rotation through the bond of chiral carbon and least priority group, which is $\ce{H}$ here (though $\ce{C-H}$ bond). Accordingly, the rotation for the first compound is clockwise as shown in the image, thus it carries (R)-notation. Since chiral carbon is 2 according to the IUPAC rules, the complete notation is (2R)-. Similarly, the mirror image is caring (2S)-notation since it has counterclockwise rotation as depicted in the image. Therefore complete names of these two isomewrs are (2R)-2,3-dihydroxypropanal and (2S)-2,3-dihydroxypropanal (Note that these isomers are called enantiomers).
In addition, if a $\ce{H}$ in $\ce{C}$3 is replaced by another different group, say it is $\ce{CH3}$ group, it will also become chiral and can have either (S) or (R) rotation. Suppose you replace $\ce{H}$ in $\ce{C}$3 of (2R)-2,3-dihydroxypropanal by a $\ce{CH3}$ group making it has (S) rotation. By addition of $\ce{CH3}$ group make it 2,3-dihydroxybutanal. With correct stereochemistry, its correct name is (2R,3S)-2,3-dihydroxybutanal.