What does floating source mean in this context?
As commented above, it is a constant current sink modulated by VSIN. I am going to assume the op-amp is ideal (inputs consume no current), and Q1 is ideal (base consumes no current-but op-amp loop account for that anyways).

Then if so Ic=Ie. Ie = \$VDC+Vsin/Rsense\$. The value of RL does not count in terms of current flow as long as it is below the maximum value for Vsense loop to work. If RL is open or 'infinite' the feedback loop is broken and the op-amp output will be close to the Vcc rail.

If Rsense is too high a value then Q1 could reach its Vce cutoff value and Ic and Ie = ~zero volts. If Rsense is too low then Ic will reach the safe limit of Q1, with only the value of RL to limit the current so Q1 is not destroyed by getting too hot.

RL 'floats' in the sense that it can have an extreme range of values yet Ic does not change. RL 'sees' only a voltage change across it as it changes values to keep the current constant if the value of RL changes. As it rises in value so does the Vdrop across RL, until it reaches Vcc-Vce-Vdrop of Rsense. That is as high as RL can go and have a stable circuit. If RL is shorted or very low ohms, nothing bad happens as the servo-loop keeps the current locked in based on Ie = \$VDC+Vsin/Rsense\$.

A dynamic current change is caused by Vsin, but it is not defined as to amplitude or frequency. One can only assume it is not enough to break the feed-back loop of the op-amp and Q1.