To be honest the whole airbus logic is pretty hard to understand. For someone trying to get how it works with no formal airbus training, no experience in airbus aircraft and (at 14 years old) probably no experience in any aircraft at all, I can completely understand that the responses I and others post are somewhat difficult to digest.
Ok so here we go!
You have still not told me what altitude you are at, and whether you are in ALT mode or DES mode.
Notwithstanding.....
Say you have passed BANOX on the way to LORNI.
Say you are in the descent, passing 4000ft.
You have selected 2500 in the FCU alt window
There are 3 options for the descent... Managed, Open, Vertical Speed/FPA
Option 1. Managed descent (DES) -
Below the DES (green) on the FMA will be ALT magenta, and the altitude on pfd will be 3000 magenta. As the aircraft captures the altitude you will get ALT CSTR* then ALT CSTR and it will maintain 3000. When you get ALT CSTR, you will also get DES blue, indicating that after LORNI it is going to continue descent to 2500. On passing LORNI, DES will activate (become green), with ALT blue and 2500ft blue on the altitude tape.
Option 2. Open Descent (OP DES)
Below OP DES (Green) on the FMA will be ALT blue, and the altitude on the pfd will read 2500. The aircraft will ignore the constraint and descend straight to and maintain 2500
Option 3. Vertical Speed or FPA (V/S or FPA)
Same as for Open descent, it ignores the constraints and goes straight to 2500.