Solar time is time measured with respect to the Sun.
Apparent solar time is based on the daily motion of the true sun, in other words, the actual position of the Sun in the sky. This is the version of solar time that a sundial shows, but it runs unevenly, for the two main reasons that Earth's axis is tilted with respect to its orbital plane and Earth's orbit is elliptical rather than circular. A better version of solar time is mean solar time, which is used for accurate timekeeping, including all civil timekeeping, is based on how the Sun would move in the sky if Earth had no axial tilt and had a perfectly circular orbit and there were no other irregularities in Earth's motion. The mean sun is a fictitious body that moves eastward in a circular orbit along the celestial equator, making a complete circuit with respect to the vernal equinox in a tropical year. It is the moving point chosen in defining mean solar time.
Solar time loses about 4 min a day compared with sidereal time because of Earth's orbital movement.