When declaring public variables what doe sit mean witha question ark

I have below public variables, with datetime and int with a ? mark, what does it mean.
Thanks a lot for the helpful info.

class DocRow {
public string logType { get; set; }
public string file_name { get; set; }
public string DM_title { get; set; }
public DateTime? recv_dt { get; set; }
public DateTime? cut_off { get; set; }
public int? retensionid { get; set; }
public string bin { get; set; }
public string folder { get; set; }
public string username { get; set; } } }