Something slender is slight, or skinny. A young colt is slender compared to the full grown horse it will grow into.
Slender can be used in both positive and negative ways. If someone tells you you have a slender figure, you'll probably be happy, unless you're going for muscle-mass. A slender portion of potatoes will not cheer a hungry growing boy. If you don't know much about something, you can say your knowledge of it is slender. And if you become an artist, get used to living on a slender budget. Now that's a nice way to put it.