The Secret Notes are a part of the “A Winter Mystery” quest that the player receives in the Winter of Year One.
The quest is activated by finding Krobus, the shadow figure, and retrieving the magnifying glass from him.
Once the quest is active, the player has the opportunity to find 25 different notes left by various residents of the valley, though not all of them are signed.
You can find them by doing a variety of different tasks from resource collection to artifact spot mining.
What is Secret Note 14 in Stardew Valley?
Secret Note 14 is one of the notes that includes only text rather than an image.

I hid something behind the community center.
– Secret Note 14 Stardew Valley
It’s relatively straightforward, although there’s no indication of who the “I” from the note actually is.
Some fans speculate that it’s Mr. Qi, the arbiter of many mysterious quests in the game, though there’s no proof to support this.
Secret Note 14 Walkthrough
Secret Note 14 requires the player to go behind the community center at the top of the Pelican Town map.

If the player uses a pickaxe or hoe against the part of the righthand wooden fence hidden by the roof, they can collect a Stone Junimo statue. This statue can be placed around the farm as a decoration.