Methotrexate is a "folate antagonist" since it acts on an enzyme called dihydrofolate reductase, a pathway enzyme necessary in the folic acid pathway. Hence they prescribe a folic acid supplement to counteract this.
As Sqad has pointed out, MCH is a measure of the haemoglobin contained within a red cell, but does not really convey much information on its own other indicators, like MCHC and most importantly, MCV and Hb are important here to assess the relevance of a raised MCH.
From a functional perspective, they will be monitoring WBC/Neutrophils levels and platelet levels principally.
ALT is an enzyme synthesised within the Liver. Methotrexate can potentially harm some liver cells, so ALT is a way of monitoring that. For a result to be clinically significant, it would be a significant rise, maybe a 2fold rise from normal.