Small plastic pots located in
public places to encourage
charitable types to deposit their
loose change to help a
good cause.
Typically, they will be to collect money for medical research or to help under-privileged groups of people.
Occasionally, these are held by well-meaning individuals who try to attract your attention by rhythmically shaking the pot. By this, they are trying to say "Please give some money to a worthwhile cause" but the message they actually convey is "I am holding a large pot of money. Mug me. Mug me now.".