stream2 stepsize meaning ?

Hey there ! Quick question about the "stream2" function. I don't understand what represents the "stepsize" option. I mean, given that I provide a velocity field (in meter/second in my case) to the function, the step should be something in seconds but it appears to be quite something else related to my grid maybe.

My goal is to connect the streamline the function return with some timeline. Maybe it's not possible ?

My code for now :

XX=ncread('20150912-GLOBCURRENT-L4-CUReul_15m-ALT_SUM-v03.0-fv01.0.nc','lon');
YY=ncread('20150912-GLOBCURRENT-L4-CUReul_15m-ALT_SUM-v03.0-fv01.0.nc','lat');
UU=ncread('20150912-GLOBCURRENT-L4-CUReul_15m-ALT_SUM-v03.0-fv01.0.nc','eastward_eulerian_current_velocity');
VV=ncread('20150912-GLOBCURRENT-L4-CUReul_15m-ALT_SUM-v03.0-fv01.0.nc','northward_eulerian_current_velocity');

[mx,my]=meshgrid(XX,YY);
mu=griddata(XX,YY,UU',mx,my);
mv=griddata(XX,YY,VV',mx,my);
XY=stream2(mx,my,mu,mv,-72,34.0,[1 1000]);

load coastlines;
figure();
plot(coastlon, coastlat);
hold on;
streamline(XY);
quiver(mx,my,mu,mv,'linewidth',2);

The velocity file can be found here : github/quai20

Many thanks for your help.