If a man is called effeminate, the speaker means his looks or behaviour seem, to them, like a woman's: a soft voice, careful movement, pretty clothes, a face or manner that is not “rough.” In older English this was a common, often sharp, judgement. It told a man he had failed a test of being “manly.” It is an adjective, and it is aimed at men and boys, not at women. A woman can be feminine; that is a normal word. Calling a man effeminate has very often been a way to put him down, including as a way to insult gay men. Though actually, the old idea — that there is one right way for a man to walk, talk, and feel — is much neater than how people really live. The word is now dated in careful speech, and many teachers would tell you to avoid it about a real person. You will still find it in older writing, and in discussions of those old attitudes. There is also a rare, old verb to effeminate, “to make someone less manly,” which you are unlikely to need. Formal; often unkind.