This is a way of asking for something. Depending on the situation, it can seem either polite or angry. Here are some situations in which it seems polite:

I'd appreciate any change that you can spare.

I'd really appreciate it if you could pick Manny up from daycare and watch him for a while this afternoon.

Here are some situations in which it seems angry:

I'd appreciate it if you'd tell me before doing something like that.

I'd appreciate a little more effort in practice from you.