Suppose a particle A is travelling in east direction with velocity of x m/s and another particle B is travelling with velocity y m/s in the west direction. Why does the the particle B appears to move towards A with a velocity of x+y and not just y m/s?
The top diagram shows the velocities in the lab frame. Particle $A$ is moving east at speed $x$ and particle $B$ is moving west at speed $y$. I'm taking the east direction to be positive, so the velocity of $A$ is positive and the velocity of $B$ is negative.
To find the velocity of $B$ relative to $A$ we have to transform to the frame where $A$ is stationary, and we do this by adding the velocity $-x$ to everything as shown in the middle diagram. Then the velocity of particle $A$ is $v_A = x + (-x) = 0$, so this makes particle $A$ stationary as we require.
And as the bottom diagram shows, when we add $-x$ to the velocity of article B we get $v_B = -y + (-x) = -(x + y)$. That's why the velocity of $B$ relative to $A$ is $-(x+y)$.
Suppose body $A$ is going to the right with a speed $x$ and another body $B$ is going to the left at a speed $y$.
The motion of the bodies can be represented as vector diagram 1.
To both motions add a velocity to the left of equal magnitude to that of the velocity of body $A$, ie stopping body $A$, as shown in vector diagram 2.
On adding the two vectors body $A$ is at rest and body $B$ is moving at speed $x+y$ to the left as shown in vector diagram 3 and this is the velocity of body $B$ relative to body $A$.
In symbols let $\hat l$ and $\hat r$ be unit vectors in the left and right direction such that $\hat l= - \hat r$.
Step 1 - The velocity of body $A$ is $x\hat r$ and than of body $B$ is $y\hat l$.
Step 2 - To both motions add a velocity to the left of equal magnitude to that of the velocity of body $A$ $(x\hat l)$
Step 3 - Velocity of $A$ is $x\hat r + x \hat l = x(-\hat l) + x \hat l =0$ and velocity of $B$ is $y\hat l + x \hat l = (x+y) \hat l$ and this is the velocity of body $B$ relative to body $A$.
As it is already answer by so many but i want to add one more point ,THAT IS -change in distance of B with repect to time will be equal to ym/s if you take your frame of reference as origin according to origin it velocity changes because it s distance with respect to time change equal to y=m/s but now yiu change frame of reference that is A so the velocity will be according to the observer A hence he feel that you are going (x+y)m per second ,yiu can add sign according to wether they are approaching each other or going away from each other