It means that when you put your baby in the cot/moses basket/cradle, you lay them (on their back) with their feet touching the foot of the cot. This is so they can't wriggle down under the covers and suffocate.
alternatively, use one of these new(ish) baby sleeping bags and save yourself the worry altogether...they're great, esp. for babies who are active at night and would easily kick covers off etc.