Meditation is not concentration.
Concentration implies a form of tension, a focus on one single thing and an exclusion of everything else. It is the mind narrowing itself.
Therefore, concentration can & will be disturbed by hungry mosquitoes, by your neighboor playing guitar at 11pm, by the cat coming to sleep on your lap, and of course by your thoughts and emotions.
Concentration is an act of will, it is the process through which one is saying to all the things mentionned above “No. You don’t exist. There is only the breath. There is only awareness”
Concentration is a fight against the world of forms and that fight can never be won. It can go on as long as you have will power, but you can never win. Eventually, anger will arise, frustration will arise. Eventually, something that will throw you off-guard will happen and life will not hesitate to punch as hard as possible (it’s for your own good after all).
Because concentration is a form of tension, a contraction of the mind, it cannot be maintained. Maybe you can maintain it throughout the day if you are really dedicated but when you go to bed, everything will be lost. You will have to begin anew the next day. And the day after, etc. It will create a fatigue.
Meditation is not concentration.
Meditation can never be disturbed because it is all inclusive, it is all welcoming, it is pure awareness watching. It is not picking. It is not saying “there is only the breath.” It is simply there, choiceless, available. It will begin by watching the breath for example, but then noise on the outside will appear, it will include it in the witnessing while staying aware of the breath.
Meditation is not concerned with objects & forms, not really. It simply notices that which is and by doing so it penetrates deeply into the nature of things. It penetrates deeply in their essence, in their interconnectedness, without ever becoming tied & attached like concentration would. This is how understanding comes, this is how insight is born.
Because meditation is your very nature, it can be maintained forever. There are people who are meditative even during sleep.