Is this an internship for a generalist consultant role, or something more technical? If it is the former, data sanitisation should be some basic tasks such as cleaning up data so it's in a consistent format, removing corrupted or unrelevant data, collapsing and summarizing data based on certain criteria. If the internship is for a more technical role, then it's probably a different story. Data sanitisation will be done at a much larger scale using more complicated tools.
Basically, consultants sometimes receive messy data, from sources like a client or a third-party market research company, for example. The data has to be cleaned up first before it can be fed into Excel, R, Tableau or some other software for data analysis. The data sanitisation you see on the job description essentially describes that cleaning process.
Hope this helps.
EDIT: Ben is correct that data sanitisation refers to removing confidential, sensitive or client-identifying info from data. I misunderstood the context when first replied.