This pattern expresses that someone does something all the time, to the exclusion of other things, and it often carries a negative or critical tone. It can be used for habits that annoy the speaker: ゲームしてばかりいる (he's always playing games). It can also describe an ongoing state that is unchanging, like 同じことばかり言っている (you keep saying the same thing). When the speaker is complaining, the nuance is 'you never do anything else!' It can also be used for oneself, with a sense of regret or apology: 最近、寝てばかりいる (lately I've just been sleeping). In casual speech, ばかり is often shortened to ばっか, but that is informal. The いる part can be omitted in casual speech, leaving just てばかり, as in 食べてばかりだ.