What is the Kosher wine that is very fruity, almost like grape juice?
It isn't. What you are thinking of is the old stuff that used to be all that you could get that was kosher. Wines like Mogan David, etc. They were the way they were because when people first came to the US, most of them were on the east coast where the only grapes were things like Concords. These grapes do not make good wine and require the addition of sugar, hence the sweet sticky wines that were produced.
Now there are much better choices in Kosher wines. Hagafen, Baron Herzog, and Covenant all make good California wines that are Kosher. BTW, there are many Israeli wines, but not all of them are Kosher. Some Israeli wines to consider are the various ones by Carmel including the Private Collection wines, and Regional Collection wines, and Dalton Reserve wines. Obviously there are others, but these are ones I've had and enjoyed.
Chateau Gascours and Domaine Barons de Rothschild both make a kosher version of their Bordeaux that is great. Other kosher Bordeaux are made by Delagrave and Château de Montage.
Ond of my favorite Kosher wines is Spanish, Capçanes Montsant Peraj Ha'Abib Kosher. The 2001 was wonderful and the 2003 is pretty good too.
Both dinwiddie and RicRios are right in so far as they go (or don't go).
The correct answer to your question is CONCORD -- whether it's bottled by Carmel, Manischewitz, Mogen David, Herzog or anyone else. Concord. Most Kosher wines (in the U.S.) have traditionally been produced from Concord grapes, a variety native to North America (Vitis labrusca). Think Welch's with alcohol.
dinwiddie is quite correct in ponting out there are numerous Kosher wines available in the U.S. produced from "wine" grapes (V. vinifera) -- in other words, from grapes like Cabernet, Chardonnay, Pinot Noir, Riesling, Chenin Blanc, Merlot, etc.
But the wine you're thinking of is Concord.