Musical theory can get pretty technical and mathematical. To be able to read music, you have to be able to count, it's all sequential fractions.
Each bar is worth a number, e.g 2/4, 3/4, 4/4, 6/8, etc, etc, and each note within that bar is worth a certain 'value'. Semibreves, minims, crotchets, quavers, semi-quaver, demi-semi-quavers, etc, are worth, e.g. 1, 2, 4, 1/2, 1/4, 1/8, 1/16, etc, etc.
This is a crap explanation, it's explained better here!
For some people this comes naturally, and for others it does take a fair bit of effort. And likewise, some people are extremely technically proficient, but not necessarily artistically/interpretatively brilliant, IYKWIM.