in Christianity this phrase would mean: to have sins partially forgiven, so that you would not be doomed eternally, its like a partial remission of what you are due. I don´t know enough about Buddhism to say exactly what is meant here. However when the words are used outside of religion they generally mean to become recognised, for example, if you pass an exam with merit, you pass it, and there´s a little extra recognition there to show that you have done more than the minimum.

Without further context, I would assume that "merit" in this instance means "a good reputation" or "good fortune/blessings" "To obtain" means "to receive" or "to earn" something. Therefore, " to obtain merit" likely means something along the lines of "to receive blessings and good fortune".