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Maths- M1 june 2001 question 7b

Can someone explain to me what it means by "horizontal speed".
The actual answers to the question can be found online BUT I'm very confused on the context because what is "HORIZONTAL SPEED"?

The rescue party moves at a horizontal speed of 5 km h-1
. The leader of the party
wants to give the walker and idea of how long it will take to for the rescue party
to arrive.
(b) Calculate how long it will take for the rescue party to reach the walker’s
estimated position.

W1 - Where the Walker is estimated to be for this part of the question.
O - origin, Where the rescue party began their journey