Lubrication
In preparation for sex, a woman's vagina will create lubrication. This is to make sex less likely to harm the vagina. I read that vaginal lubrication is very similar to saliva.
If there's not enough lubrication, either because the woman is not sexually aroused or she's old, do not use a substitute like butter, oil or Vaseline. You may as well poor Vaseline down your throat to help you swallow steak. You can buy water-based lube at the drug store or some grocery stores. Or you can use your own saliva, which, as stated above, is very similar.
I hear that a woman's lubrication may decrease with age.
If you should happen to create lubrication against your will, such as during rape, it is not your fault and it does not mean that you wanted it to happen or you consented.
I'm also told that sometimes the vagina discharges? some liquid, but it is not the same as lubrication and is not in preparation for sex.
If you hear about a woman getting "wet" in anticipation of sexual activity, this is what they mean.