The 'H' stands for 'higher', which indicates the difficulty, commitment level, and demands of a subject. Most students take a combination of H1 and H2 subjects. H3 subjects are optional, and are often reserved for high-achieving students with a passion for the subject. All A-level subjects can be taken at any of the 3 levels.
H1: Less demanding study level. Fewer topics and less depth. Less weightage in rank point calculation.
H2: Where most core subjects are taken. Higher weightage in rank point calculation.
H3: Higher level for those keen to pursue research or deeper study into a subject. Not included in rank points.