matlab command to smoothen a curve

hi, i have a curve which has a little noise. i want to smoothen the curve using a low pass filter. what is the matlab command for doing this? i came acrosss the function : y=filter(a,[1 a-1], x) where x is input values what is the value of a to be set? is it to be set to the maximum value to be passed?

thanks in advance.

regards, shiksha