When you have enabled (checked on) the "Always accept better lines" feature, it means that if your selected line changes in your favor with greater payout result, the bet slip automatically accepts your wager.
If you do not wish to use this feature and would like to receive a line change notification message, please make use of the check box next to "Always accept better lines" to turn it off.
When you have disabled (checked off) the "Always accept better lines" feature, it means that if your selected line changes, the bet slip will always notify you before accepting your wager.