well this is my sense of explaning it...
Echhi or others may call it "fan service" is an anime genre meaning the anime may consist of some minor nude and sexual scenes but its not hentai at all, its just used as an additional spice to the anime to make more exciting, some examples of echhi scenes may contain something like this..
1.When you se a girl naked during a bath
2.When girls come to the boy trying to seduce him
3.When there a scene which there trying to have sex
so basically echhi is when you see girls doing naughty stuff hehehe
well this also applies in guys, but its a lot better when girls are doing the naughty stuff