about, approximately
ばかり is used after numbers to express approximation, meaning 'about' or 'approximately'.
This use of ばかり follows a quantity expression to indicate that the number is approximate, not exact. It is similar to ぐらい or ほど, but ばかり here sounds a bit more literary or old-fashioned. In everyday conversation, you are more likely to hear ぐらい. However, you will encounter this ばかり in written Japanese and formal speech.
Note that ばかり has another common meaning: 'only' or 'nothing but', as in 漫画ばかり読んでいる (I read only manga). That usage attaches to nouns or verbs in different patterns. Do not confuse them.