Perhaps you know man, as in: Man kann hier schwimmen / One/You can swim here. This is just a way to generically refer to unspecified people.
But man can only be used when it is the subject of a sentence (and hence only in the nominative case). However, for the accusative and the dative case, a replacement exists: the masculine form of einer (singular), i.e. einen in the accusative and einem in the dative case. (There is no alternative for the genitive case.)
This is what you are looking at here. Es schadet einem nicht simply means that it doesn't hurt one/you. An example with the accusative would be Ein Rechtsstreit kann einen viel Geld kosten (a court proceeding can cost you a lot of money).