Welcome! The Virtual Seed Tray is a utility created by Borg that allows you to store more seeds. You can find it (and instructions for using it) in the Guides section for Plant Tycoon. There are two versions, one with no spoilers and another that contains spoiler information. Since you're new to the game, you might not want spoilers yet, so here is the link to the first one.

At startup, the game shows helpful tips. You can find the same tips by clicking on the faucet in your greenhouse.