What is the exact meaning of the lifetime histogram of gmx select?

GROMACS version: 2018.7
GROMACS modification: No

Dear Community,
I am wondering what is the meaning of the -olt in gmx select? I am looking at the solvent molecules in the proximity of a crystal. The histogram for the water molecules show a descending trend converging to a value around 100 when -olt option is employed. Does this mean that only 100 solvent molecules are “repeatedly selected” at each frame of the trajectory? Please see the figure…

Thank you for your responses.