What it means
A long position can lose at most what was put in, because price can't fall below zero (leverage aside).
In practice
Going long is the natural direction for most people: you buy, wait and sell higher. In stocks it also means owning the asset, with any dividends that come with it.
Traders look for longs when the market is making higher highs and higher lows, ideally buying a pullback into support rather than chasing a price that has already run. The stop goes below the level that would prove the idea wrong.
What to watch for
Being long in a falling market because price looks cheap is one of the most expensive habits in trading. A price that has dropped a long way can drop further; wait for the structure to turn first.