Progress bar

Progress bar is in the progress of the level.

One flag
A difficulty of easy is known. Most Last Stand Levels, and each level in Player's House has only one flag.

Two flags
Usually, most levels have two flags, about a difficulty of Medium. Modern Day and Jurassic Marsh have most levels with two flags.

Three flags
A difficulty of hard is known, three flags begin in an Endless Zone or Big Wave Beach - Day 2 or Pinata Party. Most levels in Lost City have three flags.

Four flags
Very hard difficulty. Starting in Pinata Party, Endless Zone, or Big Wave Beach - Day 9. Big Wave Beach and Frostbite Caves are the only worlds that more than one level has four flags.

Trivia

 * There are Waves in Plants vs. Zombies 2, which makes the progress bar sometimes slow or fast.