Prophet Zephaniah pronounced divine judgement upon Jerusalem for it's many sins - chiefly idolatry. Jerusalem obeys none, accepts no correction, does not trust in the LORD, her officials are corrupt, rulers are like wolves, prophets are unprincipled, priests are profaned and do violence to the law.
Rebellion against the LORD and corruption has entered every sector within the society. The LORD has no choice, but to wipe the city out.
The prophet speaks about those survivors - those few humble and faithful, who will remain while the nation is purged and purified. The remnant will trust in the LORD, do no wrong, tell no lies, and nobody will make them afraid.
The LORD is with them, caring and protecting them - it is a reason to rejoice. He is pouring out His love upon them. When He gathers and brings them home, He will honor and praise them, and restore their fortunes.