The ball is thrown horizontally at 10 m/s.
Its horizontal speed doesn't change, but because of gravity,
it picks up vertical (downward) speed at the rate of 9.8 m/s
every second.
So at the end of one second, the ball is moving ...
horizontally at 10 m/s
and
downward at 9.8 m/s .
All together, it's moving in kind of a slanted direction,
at a speed of
√ ( 10² + 9.8² )
= √ (100 + 96.04)
= √ (196.04) = 14 m/s (rounded)