I was curious about what does Pokemon go nearby panel tell us. Its not guaranteed that if I linger in that area I will find same Pokemons shown. Also what are grayed out Pokemons for?
Before the first update, the 'nearby' tab (or more formally known as the tracker) used to show nearby Pokemon with either 1, 2, or 3 footprints. These footprints would show how far a nearby Pokemon was to you, with 1 footprint being the closest and 3 being the furthest.
Since the most recent update, Niantic has removed the footprint tracking features of the tracker, due to its inaccuracy. Now the 'nearby' tab just shows Pokemon that are nearby without any metric of how far away they are. It is a little unclear, but many speculate that if a Pokemon appears on the 'nearby' tab that it is within 200m of your position.
Here's a related question that shows how the tracker worked before the update: Why should I "watch" a nearby pokémon?
As for the greyed out shapes on the 'nearby' tab, those refer to Pokemon that you haven't caught yet. When you first start the game, all of the Pokemon on the tracker appear as grey shapes (except for who you picked as your starter) until you catch them. Once you catch a Pokemon of that variety, it will appear colored-in on the tracker next time you see it. This is especially important because uncaught Pokemon species reward you with an extra +500xp when you catch them, so you generally watch to catch Pokemon that you see greyed out on your tracker.