What does X + Y = Z mean?

If you have turned on "To-Hit Rolls" in your Feedback settings, the game will display the rolls on each attack. The roll will display:

X + Y = Z

Where
X = base to hit roll (1-20)
Y = any bonuses/penalties to hit
Z = total to hit roll

Y is the total aggregate value of all bonuses and/or penalties that are applied to the character's to hit roll. This value is also usually reflected in the difference between the character's THAC0 and base THAC0 on the character page. However, certain bonuses or penalties will show in the value Y that don't show on the THAC0s on the Character Record, such as off handed attack penalties. Weapon versus armor type bonuses and penalties are also applied to Y.

You might want to read further on this topic.