Albert Barnes Commentary 2 Samuel 18:3

"But the people said, Thou shalt not go forth: for if we flee away, they will not care for us; neither if half of us die, will they care for us: but thou art worth ten thousand of us; therefore now it is better that thou be ready to succor us out of the city." — 2 Samuel 18:3 (ASV)

Succour us out of the city - David, with a reserve force, would hold the city, and either support the troops in case of need, or receive them within the walls should they be compelled to flee.