I once heard that everything is relative, except the speed of light,
which is absolute...

is that true really? if living things cannot see, and therefore not
be able to see light, then maybe we will think everything is relative,
as we don't know light exists.

we often hear that when the speed of an object increases (close to the
speed of light), then its mass increases, and its length decreases...
is it "absolute speed" here or "relative speed"? is it true that only
an observer which sees that object moving close to the speed of light
will measure that increase of mass and decrease of length, but let's
say there is an ant on that moving object, the ant won't measure the
object having increased mass and decreased length.

(once i heard that an object cannot move faster than the speed of
light, as the mass will approach infinity and therefore no energy is
enough to make it move faster... so is that not true, since its mass
according to the ant is unchanged).

sorry can things be discussed as if it is explained to a 10-year
old? Thanks very much for answering to such a simplistic question.