A 'true friend' is the opposite of a 'fair-weather friend'. A fair-weather friend only hangs around for the fun times : if you you become sick, poor or lonely, a fair-weather friend will abandon you.
By contrast, a 'true friend' is a friend for life. If you're ill, if you're in trouble, if times are bad, a true friend will always be there for you. A true friend will never betray you or let you down, and will support you and defend you no matter what.