Why is my low pass filter magnigying my signal?

Why is my low pass filter magnigying my signal?

I am a complete beginner at MATLAB and signal analysis so I don't really
understand the fourier mathematics behind signal filtering. I took my Ipod
Touch and used an app that records the accelerometer data (at 50Hz
sampling rate) to a CSV and i loaded that CSV data into matlab. I am now
trying to apply a low pass filter to the signal to get rid of the noise
that occurs when from small changes in acceleration (so i am left with
just the large signals of acceleration).
I read a bunch about signal filtering in matlab today and i own the signal
processing toolbox, but i am still very confused on what certain variables
mean. For instance the function filter(b,a,data) keeps coming up and i
know that a and b are vectors of coefficients but i have no idea how these
effect the filter or how to create these values based on what i know about
my signal/what i want done with it.
Where should i go to learn the basics of signal processing if i don't have
a strong background with fourier analysis? And how should i go about
creating my low pass filter in matlab? Thanks!
Here is the sample data i am working with.