This is an amusing joke that seems to imply opposite meanings - making it a double entendre. If Mary only cares about their presence, it means she just wants her guests to show up, but the word itself sounds a lot like presents, implying the exact opposite of what was written.
When asked by her co - workers whether they should bring a gift to her birthday party , Mary replied , ' ' You should know that all I ' m interested in is your presence . ' '