What is a long/short fund?
A long/short fund buys stocks it expects to rise (going long) and sells short stocks it expects to fall. The shorts can make money when the market drops, cushioning the losses on its longs.
Two numbers describe such a fund. Its gross exposure is its longs plus its shorts, as a share of its capital: how much it has at stake. Its net exposure is longs minus shorts: how much it gains or loses when the whole market moves. A fund that's 100% long and 60% short is 160% gross and 40% net long.
Some pair trades directly, buying one company and shorting a competitor so only the difference between them matters.
What 13Fs show
Only the longs. A long/short fund's 13F can make it look far more bullish than it is, and a stock it owns may be there to hedge a short you can't see.