What does it mean when I'm told that I "indulge" my friend and yet get nothing in return from the friendship?

Well, there's not much info to go on but I think I agree with Venus, above.
But the question you have to ask yourself is "what DO I get from this relationship?" That doesn't have to feel selfish, and it doesn't have to be 100% equal (giving & getting) but try thinking like this:
- is this friendship mostly just stressing me out?
- do I enjoy spending time with ___?
- when I'm with ____, is all she does complain or ask for my help?
- am I being honest with this friend? (not just humoring her even though I really don't agree inside)

The "indulging" part means maybe you're letting the friend make all the decisions (maybe that's ok with you or maybe this other friend thinks she's using you or pushing you around).
Or it could mean you're humoring her but not being honest, for example, If she's complaining about how her boyfriend was mad because blah blah blah... maybe you're thinking "well, actually I'd be mad too, you should've blah blah" but what you're SAYING to her is "Yeah, you're right, what a jerk he is!"
(just an example of indulging someone in a way that's actually harmful)

some ppl are really manipulative, and use everyone they know to get their needs met. others of us are pure and kind and, yes, indulge these ppl.

some people are givers, others are takers. you're a giver, your friend is a taker.

you are the giver and not the taker and tend to get walked on by your friend as being the doormat! I would find a better friend, that is just me!

I think that....The person who told you that.....Is not seeing it with your eyes......No one but you....knows what you get out of that friendship....So please do not judge YOUR friendships by what others say....Only you can say when you are being used...And no one else.

There was a time when friends of mine told me that I should get a divorce...That my husband was using me....And that I was indulging him......But I knew the real him.....and the whole story.....and knew that the one that they saw...Was just a bad woman bring out the worse in him...

It means that you have a friend who is a narcissist.....or at least has narcissistic tendancies....any real friend would naturally give, not just take all the time.