The beer finish stuff is a range and is best forgotten.
Where it will finish depends on a lot of factors - in the case of kits, ingredients is mostly it.
Straight kit + 1 kg white sugar or dextrose should finish around between1.010 and 1.008. A couple of points higher isn't a big deal if you've given it time and love but 3-4 points or more suggests that the beer is not ready. Yeast can stall and bottling early can mean bottles explode.
If you start adding maltodextrin or malt extract to your brews you can expect them to finish higher. Higher gravity beers also will often finish higher. If you add a dry enzyme, expect it to go a fair bit lower.
There are ways to kick off a stalled yeast and ways to find out where beer should finish but right now, for basic kits, no extras - look at 1010 (actually 1.010) as a rough guide. 1008 - 1012 and stable for a few days at the right temperature. Some kits suggest 1006 but I never had one drop that low.
As the gravity drops, the hydrometer sinks and thus gives a lower reading.
Be aware that gravity reading is affected by temperature so hot/warm reads lower. There are calculators you can use to correct this but for now, try and stick to 18-20 degrees for fermentation and for readings.
1.000 is what pure water will measure at room temperature (and measuring water at close to 1000 is a good way of making sure your hydrometer is accurate) so higher than 1.000 shows there is some sugar in solution. Some of this won't get fermented by beer yeast which results in body and some flavour in your beer.
