What is the meaning of the English phrase 'a friend you can count on'?

"A person who is there when you need her/him" is the meaning of the English phrase "a friend you can count on".

Specifically, the phrase "count on" can be replaced by the phrase "depend upon". It means that the person will not disappoint whenever participation or support is needed. For example, such a person will be available for an important get-together or lend an ear when a good listener is needed.