LTCG are gains on investments that you've held for one year and then you sell at a gain. Buy at 10, hold for a year, sell at 12, LTCG is 2.
Now what is important is the tax impact. Short term capital gains on investment you held for less than 1 year are taxed as ordinary income.
LTCG and qualified dividends(most stock dividends you will run into are qualified, that is all you need to know) are taxed on a different tax table than ordinary income and it is an amazing table to be on.
Ordinary income tax bracket/ LTCG tax bracket.
10/ 0%
15/ 0%
25/ 15%
33/ 15%
etc.
Notice if you are in a lower tax bracket, 10-15% brackets, LTCG are taxed at ZERO! Even if you are in the 25-35 its only 15%. Even at the highest tax bracket, almost 40%, LTCGs are taxed at 20%.