It means pretty much exactly what it says. In Matlab x1, as you create it, is an array of values representing your function at defined points. Those values are indexed as 1, 2, 3, 4, 5,..., not as the true t values of your equation. However, you can use vectorisation to assign all values at once:
x1 = uo*(1-c1.*(cos(wd1*t)-(y1*sin(wd1*t))));
Just like you do with c1, which also works with a full vector of t values.
Notice you are already using .* for your multiplication, which is what is needed for vectorised maths here since c1 and the quantity it is being multiplied by are both vectors, so this gives you element-wise multiplication and will assign all the results into x1, which will be the same length as t at the end, and that provides your link between t and x, by indexing into each.