It is thorougly discussed in this reddit post. It describes what it is and how it works, as well as how to perform it properly.
So soaking XP you have to be a certain distance to the minions that
are killed. With any other character this is easy as you're usually
right in the lane fighting them. I know with Abathur that if your
locusts or mines get the killing blow then you get XP however that is
a pretty low chance especially in early game.
How do you soak XP properly with Abathur? I keep seeing people saying
to body soak does that mean to throw Abathur out into the lane and not
in a bush near the lane? Do I hat one lane and help mow it down and
then just bounce around hatting lanes or should I actually be in the
lane with Abathur and doing damage and slappin' those minions?
There is certainly an art to know when, where, and for how long you
can go out into lane to soak.
Most importantly, your vision radius is how close you need to be to
dying minions in order to passively body-soak the XP. This aspect is
largely ignored by many Abathur players imo.