Lag generally refers to a delay in something. There are many types of "lag" in Second Life, such as lag caused by a slow or faulty internet connection, lag caused by a sim that is experiencing a "Deep-Think", and "lag" of FPS.
Sims will sometimes lag if there are a lot of avatars in them and will definitely lag if there are a lot of physical objects that are interacting with things, including avatar attachments and Animation Overrides.
Connection lag will be noticeable if you can not move your avatar or if chat you are typing in does not appear. If the packet loss bar (upper right hand corner on the left, not the right bar, the left) is red, this means there is a network issue between you and the server.
If you have low FPS, try adjusting some of your settings and disabling "Graphics"/"Graphics Details" features.