yield can be inferred from many instruments, there are three things to consider when talking about yield,

t0: moment of calculation for yield,

t1: starting time for yield period,

t2: ending time for yield period,

at t0, one collects a bunch of bond prices(spot prices), from these bond prices, one can infer yields for period from t1 to t2.

if t1 is same as t0, you get a spot yield curve, if t1 is later than t0, you get a forward yield curve, which is yield curve for a future time based on current market prices.