This simply means that the lead was acquired (created in the Marketo database) as a result of that Program. For example, if you have a Marketo Landing Page & Form which belongs to a Program, which you then link to via your website or otherwise, any leads who sign up will be "Acquired By" that particular Program.
Another common scenario is where you've sent out an Email Blast, and a lead forwards it to a colleague who then fills in the form. If this colleague wasn't already in your database, they are now, and have been "Acquired By" the Program.
Hope that's clear!